Embed frontend dist into Go binary; remove frontend path from health

- New backend/embed package embeds frontend dist/ via //go:embed
- Go binary serves frontend from embedded FS (no external files needed)
- Dev mode uses placeholder dir (dev.txt) so Go compiles; Vite handles
  frontend in dev mode
- Makefile copies frontend/dist/ to backend/embed/dist/ during build
- Removed FRONTEND_DIR env var and filesystem serving fallback
- Removed frontend path from Health API response and Status page frontend
  section (only Served boolean remains)
- Removed frontendDir from Handler struct/constructor (unused)

backend/cmd/server/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"io/fs"
 	"log"
 	"net/http"
 	"os"
@@ -10,6 +11,7 @@ import (
 	"syscall"
 	"time"
 
+	"arma3-web-server/embed"
 	"arma3-web-server/internal/api"
 	"arma3-web-server/internal/services"
 
@@ -24,7 +26,6 @@ func main() {
 	cfgDir := mustAbs(getEnv("CFG_DIR", filepath.Join(devDeploy, "serverfiles", "cfg")))
 	profilesDir := mustAbs(getEnv("PROFILES_DIR", filepath.Join(devDeploy, "serverfiles", "profiles")))
 	listen := getEnv("LISTEN", ":8080")
-	frontendDir := mustAbs(getEnv("FRONTEND_DIR", filepath.Join("..", "frontend", "dist")))
 
 	log.Printf("data dir:       %s", dataDir)
 	log.Printf("serverfile dir: %s", serverfileDir)
@@ -55,16 +56,19 @@ func main() {
 
 	r := gin.Default()
 
-	if fi, err := os.Stat(frontendDir); err == nil && fi.IsDir() {
-		r.Static("/assets", filepath.Join(frontendDir, "assets"))
-		r.StaticFile("/favicon.ico", filepath.Join(frontendDir, "favicon.ico"))
-		r.NoRoute(func(c *gin.Context) {
-			c.File(filepath.Join(frontendDir, "index.html"))
-		})
-		log.Printf("serving frontend from %s", frontendDir)
+	subFS, subErr := fs.Sub(embed.Frontend, "dist")
+	if subErr == nil {
+		if _, err := subFS.Open("index.html"); err == nil {
+			r.StaticFS("/assets", http.FS(subFS))
+			r.StaticFileFS("/favicon.ico", "favicon.ico", http.FS(subFS))
+			r.NoRoute(func(c *gin.Context) {
+				c.FileFromFS("index.html", http.FS(subFS))
+			})
+			log.Print("serving embedded frontend")
+		}
 	}
 
-	handler := api.New(settings, configMgr, modlistMgr, process, steamcmd, scheduler, streamer, dataDir, serverfileDir, modsDir, cfgDir, profilesDir, frontendDir)
+	handler := api.New(settings, configMgr, modlistMgr, process, steamcmd, scheduler, streamer, dataDir, serverfileDir, modsDir, cfgDir, profilesDir)
 	handler.SetupRoutes(r)
 
 	// Startup auto-tasks