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/internal/api/router.go

@@ -19,7 +19,6 @@ type Handler struct {
 	modsDir       string
 	cfgDir        string
 	profilesDir   string
-	frontendDir   string
 }
 
 func New(
@@ -30,7 +29,7 @@ func New(
 	steamcmd *services.SteamCmdManager,
 	scheduler *services.Scheduler,
 	streamer *services.LogStreamer,
-	dataDir, serverfileDir, modsDir, cfgDir, profilesDir, frontendDir string,
+	dataDir, serverfileDir, modsDir, cfgDir, profilesDir string,
 ) *Handler {
 	return &Handler{
 		settings:      settings,
@@ -45,7 +44,6 @@ func New(
 		modsDir:       modsDir,
 		cfgDir:        cfgDir,
 		profilesDir:   profilesDir,
-		frontendDir:   frontendDir,
 	}
 }