docs: update PLAN, CODEBASE, README with env vars, stubs, tests, atomic state

PLAN.md:
- Add SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH to env vars table
- Add testdata/ to project structure
- Add testing section (backend tests, stubs, frontend tests, make targets)
- Update process start flow with env var overrides

CODEBASE.md:
- Add testdata/ stubs to directory map
- Replace zustand with vitest/testing-library/jsdom in frontend deps
- Update CI to include test steps
- Update Makefile targets description
- Add atomic state machine to ProcessManager description
- Add env overrides and atomic state to design decisions
- Update data flow with env var overrides

README.md:
- Add SERVER_BINARY, SERVER_PARAMS, STEAMCMD_PATH to env vars table
- Remove FRONTEND_DIR (embedded frontend)
- Update Development section with make targets

CODEBASE.md

@@ -40,10 +40,13 @@ arma3-web-server/
 │   │       ├── modlist_manager.go   # Modlist CRUD against data/modlists/*.json
 │   │       ├── modlist_parser.go    # HTML Arma Launcher preset parser + renderer
 │   │       ├── mod_manager.go       # Workshop + local mod discovery + usage map
-│   │       ├── server_process.go    # Process lifecycle, arg builder, mod path resolver
+│   │       ├── server_process.go    # Process lifecycle, arg builder, mod path resolver (atomic state machine)
 │   │       ├── steamcmd.go          # SteamCMD manager (UpdateGame, DownloadMod, DownloadMods)
 │   │       ├── scheduler.go         # Cron-based scheduled updates
-│   │       └── log_streamer.go      # Pub/sub fan-out for stdout/stderr via channels
+│   │       ├── log_streamer.go      # Pub/sub fan-out for stdout/stderr via channels
+│   │       └── testdata/            # Stub binaries for testing
+│   │           ├── arma3server_x64  # Server stub: logs args, heartbeat, -t N exit, RPT writes
+│   │           └── steamcmd         # SteamCMD stub: fake downloads, creates mod dirs
 │   ├── go.mod / go.sum
 │   └── serverfiles/                 # Default SERVERFILE_DIR (created at startup)
 ├── frontend/                        # React SPA
@@ -107,11 +110,14 @@ arma3-web-server/
 | `tailwindcss` | styling | ^4.3.2 | Utility-first CSS |
 | `@monaco-editor/react` | editor | ^4.7.0 | Monaco code editor for .cfg files |
 | `@xterm/xterm` / `@xterm/addon-fit` | terminal | ^6.0.0 / ^0.11.0 | In-browser xterm.js terminal |
-| `zustand` | state | ^5.0.14 | Declared but **unused** (no stores exist) |
 | `vite` | bundler | ^8.1.1 | Dev server + production build |
 | `@vitejs/plugin-react` | tooling | ^6.0.3 | React fast-refresh / JSX transform |
 | `typescript` | language | ~6.0.2 | Type checking |
 | `oxlint` | linter | ^1.71.0 | Linting |
+| `vitest` | test runner | ^4.1.10 | Frontend unit/integration tests |
+| `@testing-library/react` | test utility | ^16.3.0 | React component testing |
+| `@testing-library/jest-dom` | test matcher | ^6.6.3 | DOM assertion matchers |
+| `jsdom` | test env | ^26.1.0 | Browser environment for tests |
 
 ## Architecture
 
@@ -133,7 +139,7 @@ Browser -- REST/WS -->  api.Handler -->  SettingsManager    -->  data/settings.j
 - **Handler** (api package) -- stateless; receives service pointers via constructor injection. All handlers are methods on Handler.
 - ***Manager** (services package) -- stateful structs for each concern. Hold their own `sync.Mutex` for thread safety.
 - **LogStreamer** -- pub/sub per named stream (`"server"`, `"steamcmd"`). Subscribe returns a `chan string`; Stream reads from an `io.Reader` (process stdout/stderr) and broadcasts to all subscribers of that key.
-- **ProcessManager** -- manages the Arma 3 server child process lifecycle. Builds the command line from settings, resolves mod paths, monitors process exit.
+- **ProcessManager** -- manages the Arma 3 server child process lifecycle. Uses `atomic.Int32` state machine (idle/starting/running/stopping) with `CompareAndSwap` for lock-free state transitions. Builds the command line from settings, resolves mod paths, monitors process exit.
 
 ### Notable design decisions
 1. **No database** -- everything is files: `settings.json`, `*.cfg` in `$CFG_DIR`, modlist JSON files. Backup means copying directories.
@@ -141,7 +147,8 @@ Browser -- REST/WS -->  api.Handler -->  SettingsManager    -->  data/settings.j
 3. **`%command%` substitution** -- if ServerParameters contains `%command%`, the entire textarea is treated as a shell template, with `%command%` replaced by the binary path and auto-args (`-config=`, `-mod=`, `-profiles=`, `-port=`) appended. Enables Wine wrappers.
 4. **Two-tier mod resolution** -- resolveModPath checks `$MODS_DIR/@name` first (manual/symlinked), then falls back to `$SERVERFILE_DIR/steamapps/workshop/content/107410/<id>` for workshop downloads.
 5. **SteamCMD async with live streaming** -- all steamcmd operations run in background goroutines, output broadcast to `"steamcmd"` key.
-6. **Env-defined paths** -- all dirs set via env vars with hardcoded defaults, resolved to absolute paths at startup.
+6. **Env-defined paths** -- all dirs set via env vars with hardcoded defaults, resolved to absolute paths at startup. `SERVER_BINARY`, `SERVER_PARAMS`, `STEAMCMD_PATH` override defaults at runtime.
+7. **Atomic state machine** -- ProcessManager and SteamCmdManager use `atomic.Int32`/`atomic.Bool` with `CompareAndSwap` for lock-free state transitions, eliminating race conditions between concurrent Start/Stop calls.
 
 ## Data flow (example: Start server)
 
@@ -152,6 +159,8 @@ User clicks "Start" (Settings.tsx)
   --> Handler.StartServer() (settings.go)
     --> process.Start() (server_process.go)
       --> Load settings.json
+      --> SERVER_PARAMS env overrides ServerParameters if set
+      --> SERVER_BINARY env overrides platform default binary name if set
       --> If %command% in ServerParameters:
           --> Replace %command% with binPath
           --> Split into command + args
@@ -174,7 +183,7 @@ User clicks "Start" (Settings.tsx)
 
 ### Gitea Actions (`/.gitea/workflows/`)
 
-- **`ci.yml`** — Triggered on push to any branch or PR. Runs `go build ./...` and `npm ci && npm run build`. Build-only; no tests.
+- **`ci.yml`** — Triggered on push to any branch or PR. Runs `go build ./...`, `npm ci && npm run build`, `go test ./...`, and `npm test`. Build + test CI.
 - **`release.yml`** — Triggered on `v*` tag push. Runs GoReleaser v2 with `release --clean`, which builds Go binaries (linux/amd64 + windows/amd64), builds frontend via `before.hooks`, creates archives, builds + pushes a Docker image using `Dockerfile.goreleaser`, and publishes a Gitea release.
 
 ### Docker
@@ -184,4 +193,4 @@ User clicks "Start" (Settings.tsx)
 
 ### Local dev
 
-- **Makefile targets** — `make run` starts backend, `make dev` starts both backend + Vite dev server with hot reload.
+- **Makefile targets** — `make run` starts backend, `make dev` starts both backend + Vite dev server with hot reload (copies test stubs to dev-deploy, passes `SERVER_BINARY`/`SERVER_PARAMS`/`STEAMCMD_PATH` env vars). `make test` runs both backend and frontend tests.

PLAN.md

@@ -51,7 +51,10 @@ arma3-web-server/
 │   │       ├── server_process.go    # Process lifecycle, arg builder, mod path resolver
 │   │       ├── steamcmd.go          # SteamCMD manager (UpdateGame, DownloadMod, DownloadMods)
 │   │       ├── scheduler.go         # Cron-based scheduled updates
-│   │       └── log_streamer.go      # Pub/sub fan-out for stdout/stderr via channels
+│   │       ├── log_streamer.go      # Pub/sub fan-out for stdout/stderr via channels
+│   │       └── testdata/            # Stub binaries for testing
+│   │           ├── arma3server_x64  # Server stub: logs args, heartbeat, -t N exit, RPT writes
+│   │           └── steamcmd         # SteamCMD stub: fake downloads, creates mod dirs
 │   ├── go.mod
 │   └── go.sum
 ├── frontend/
@@ -91,7 +94,7 @@ arma3-web-server/
 ├── Dockerfile                       # Multi-stage: Go build -> npm build -> alpine runtime + SteamCMD
 ├── Dockerfile.goreleaser            # Single-stage for GoReleaser (pre-built artifacts injected)
 ├── .goreleaser.yaml                 # GoReleaser v2 config (Gitea release target)
-└── Makefile                         # Convenience targets: backend, frontend, build, run, dev, clean
+└── Makefile                         # Convenience targets: backend, frontend, build, run, dev, clean, test
 ```
 
 ---
@@ -106,6 +109,9 @@ arma3-web-server/
 | `PROFILES_DIR` | Server profile/save/log path | `$SERVERFILE_DIR/profiles` |
 | `DATA_DIR` | Internal data (settings.json, modlists/) | `./data` |
 | `LISTEN` | HTTP listen address | `:8080` |
+| `SERVER_BINARY` | Server binary filename (overrides platform default) | `arma3server_x64` (`.exe` on Windows) |
+| `SERVER_PARAMS` | Override server launch parameters (overrides settings.json) | `-server -world=empty -loadMissionToMemory -noPause` |
+| `STEAMCMD_PATH` | Path to steamcmd binary | `steamcmd` |
 
 > Note: `FRONTEND_DIR` was removed — the frontend is now embedded into the Go binary via `//go:embed`.
 
@@ -237,6 +243,8 @@ GET    /ws/server/rpt                # tail latest .rpt crash dump (250ms pollin
 User clicks Start
   → Settings loaded from data/settings.json
   → WasRunning set to true, saved to disk
+  → SERVER_PARAMS env overrides ServerParameters if set
+  → SERVER_BINARY env overrides platform default binary name if set
   → If ServerParameters contains %command%:
       → Replace %command% with binary path (enables Wine wrappers)
       → Split into command + args
@@ -286,3 +294,52 @@ A cron expression in the `scheduled_update` field runs game + mod updates on a s
 10. **Crash recovery** — `WasRunning` flag persists across restarts. If `auto_start_on_startup` is enabled, the server restarts automatically.
 11. **SteamCMD async with live streaming** — All steamcmd operations run in background goroutines, output broadcast to `"steamcmd"` key via LogStreamer pub/sub.
 12. **Health endpoint** — `/api/server/health` checks binary existence, directory writability, disk usage, mod counts, SteamCMD status, and frontend serving.
+
+---
+
+## Testing
+
+### Backend tests
+
+Run with `make test-backend` or `cd backend && go test ./...`.
+
+Test files in `backend/internal/services/`:
+- `settings_test.go` — SettingsManager load/save/defaults
+- `config_manager_test.go` — ConfigManager CRUD + path traversal protection
+- `modlist_manager_test.go` — ModlistManager CRUD + duplicate
+- `modlist_parser_test.go` — HTML preset parser + renderer
+- `log_streamer_test.go` — Pub/sub subscribe/unsubscribe/broadcast/stream
+- `server_process_test.go` — Process lifecycle, arg building, mod path resolution, env var overrides, stub-based integration tests
+- `steamcmd_test.go` — SteamCMD state machine, CheckWorkshopMod, stub-based download tests
+
+### Test stubs
+
+Stub binaries in `backend/internal/services/testdata/` replace real server/steamcmd during tests:
+
+**`arma3server_x64` stub:**
+- Logs binary path and all arguments to stdout
+- Prints heartbeat every 5 seconds
+- Accepts `-t N` to exit after N seconds (for auto-exit and timeout tests)
+- Parses `-profiles=<dir>` and writes RPT log entries to `profilesDir/arma3server_x64_*.rpt`
+
+**`steamcmd` stub:**
+- Parses `+force_install_dir` and `+workshop_download_item` args
+- Creates fake mod directories at the expected workshop content path
+- Prints fake download progress and success logs
+
+### Frontend tests
+
+Run with `make test-frontend` or `cd frontend && npm test`.
+
+Uses Vitest + React Testing Library + jsdom:
+- `utils/format.test.ts` — fmtSize utility
+- `components/Layout.test.tsx` — Sidebar navigation
+- `pages/Mods.test.tsx` — Mods page with mods present
+
+### Makefile test targets
+
+| Target | Description |
+|--------|-------------|
+| `make test` | Run both backend and frontend tests |
+| `make test-backend` | Run `go test ./...` in backend |
+| `make test-frontend` | Run `vitest run` in frontend |

README.md

@@ -47,8 +47,10 @@ All paths are configurable via environment variables:
 | `MODS_DIR` | `serverfiles/mods` | Local mod symlinks/copies |
 | `CFG_DIR` | `serverfiles/cfg` | Server configuration `.cfg` files |
 | `PROFILES_DIR` | `serverfiles/profiles` | Arma 3 profile and log directory |
-| `FRONTEND_DIR` | `../frontend/dist` | Built frontend assets |
 | `LISTEN` | `:8080` | HTTP listen address |
+| `SERVER_BINARY` | `arma3server_x64` | Server binary filename (overrides platform default) |
+| `SERVER_PARAMS` | `-server -world=empty ...` | Override server launch parameters |
+| `STEAMCMD_PATH` | `steamcmd` | Path to steamcmd binary |
 
 ## Automation
 
@@ -150,14 +152,23 @@ frontend/
 ## Development
 
 ```bash
-# Backend
+# Start both backend + frontend with hot reload (copies stubs to dev-deploy)
-cd backend && go run ./cmd/server
+make dev
 
-# Frontend (hot reload)
+# Start backend only
-cd frontend && npm run dev
+make run
 
-# Lint
+# Run all tests
-cd frontend && npm run lint
+make test
+
+# Run backend tests only
+make test-backend
+
+# Run frontend tests only
+make test-frontend
+
+# Build for production
+make build
 ```
 
 ## License