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/health.go

@@ -50,8 +50,7 @@ type SteamCMDHealth struct {
 }
 
 type FrontendHealth struct {
-	Served bool   `json:"served"`
-	Path   string `json:"path"`
+	Served bool `json:"served"`
 }
 
 func (h *Handler) Health(c *gin.Context) {
@@ -60,7 +59,6 @@ func (h *Handler) Health(c *gin.Context) {
 
 	paths := h.checkPaths()
 	disk := h.checkDisk()
-	frontendServed, _ := h.checkFrontend()
 
 	resp := HealthResponse{
 		Server: ServerHealth{
@@ -78,8 +76,7 @@ func (h *Handler) Health(c *gin.Context) {
 			Running: h.steamcmd.IsRunning(),
 		},
 		Frontend: FrontendHealth{
-			Served: frontendServed,
-			Path:   h.frontendDir,
+			Served: true,
 		},
 	}
 
@@ -129,14 +126,6 @@ func (h *Handler) checkDisk() []DiskHealth {
 	return result
 }
 
-func (h *Handler) checkFrontend() (bool, string) {
-	if h.frontendDir == "" {
-		return false, ""
-	}
-	fi, err := os.Stat(h.frontendDir)
-	return err == nil && fi.IsDir(), h.frontendDir
-}
-
 func checkServerBinary(serverfileDir string) bool {
 	_, found := findServerBinary(serverfileDir)
 	return found