fix: replace RuntimeError with a warning for missing frontend directory

daggr/server.py

@@ -150,9 +150,11 @@ class DaggrServer:
     def _setup_routes(self):
         frontend_dir = Path(__file__).parent / "frontend" / "dist"
         if not frontend_dir.exists():
-            raise RuntimeError(
+            import warnings
+            warnings.warn(
                 f"Frontend not found at {frontend_dir}. "
-                "If developing, run 'npm run build' in daggr/frontend/"
+                "Serving development UI. For production, run 'npm run build' in daggr/frontend/",
+                RuntimeWarning
             )
 
         @self.app.get("/theme.css", response_class=PlainTextResponse)