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)

Makefile

@@ -15,15 +15,23 @@ backend:
 frontend:
 	cd frontend && npm run build
 
-build: backend frontend
+.PHONY: copyfrontend
+copyfrontend:
+	rm -rf backend/embed/dist
+	mkdir -p backend/embed/dist
+	cp -r frontend/dist/. backend/embed/dist/
+
+build: frontend copyfrontend backend
 
 run:
 	cd backend && DEV_DEPLOY_DIR=$(DEV_DEPLOY_DIR) SERVERFILE_DIR=$(SERVERFILE_DIR) MODS_DIR=$(MODS_DIR) CFG_DIR=$(CFG_DIR) PROFILES_DIR=$(PROFILES_DIR) DATA_DIR=$(DATA_DIR) GIN_MODE=debug go run ./cmd/server/
 
 dev:
+	mkdir -p backend/embed/dist
+	test -f backend/embed/dist/dev.txt || touch backend/embed/dist/dev.txt
 	cd backend && DEV_DEPLOY_DIR=$(DEV_DEPLOY_DIR) SERVERFILE_DIR=$(SERVERFILE_DIR) MODS_DIR=$(MODS_DIR) CFG_DIR=$(CFG_DIR) PROFILES_DIR=$(PROFILES_DIR) DATA_DIR=$(DATA_DIR) GIN_MODE=debug go run ./cmd/server/ &
 	cd frontend && npm run dev
 
 clean:
 	rm -f backend/arma3-web-server
-	rm -rf frontend/dist
+	rm -rf frontend/dist backend/embed/dist