Add frontend fallback resolver

app/frontend_static.py

@@ -120,3 +120,16 @@ def register_frontend_static_routes(app: FastAPI, frontend_dir: Path) -> bool:
 
     logger.info("Serving frontend from %s", frontend_dir)
     return True
+
+
+def register_frontend_missing_fallback(app: FastAPI) -> None:
+    """Register a fallback route that tells the user to build the frontend."""
+
+    @app.get("/", include_in_schema=False)
+    async def frontend_not_built():
+        return JSONResponse(
+            status_code=404,
+            content={
+                "detail": "Frontend not built. Run: cd frontend && npm install && npm run build"
+            },
+        )

app/main.py

@@ -8,7 +8,7 @@ from fastapi.responses import JSONResponse
 
 from app.config import setup_logging
 from app.database import db
-from app.frontend_static import register_frontend_static_routes
+from app.frontend_static import register_frontend_missing_fallback, register_frontend_static_routes
 from app.radio import RadioDisconnectedError, radio_manager
 from app.radio_sync import (
     stop_message_polling,
@@ -109,4 +109,5 @@ app.include_router(ws.router, prefix="/api")
 
 # Serve frontend static files in production
 FRONTEND_DIR = Path(__file__).parent.parent / "frontend" / "dist"
-register_frontend_static_routes(app, FRONTEND_DIR)
+if not register_frontend_static_routes(app, FRONTEND_DIR):
+    register_frontend_missing_fallback(app)