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

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 |