reworded fastapi code

python/api/routers/frontend.py

@@ -0,0 +1,24 @@
+"""Frontend SPA router."""
+
+from pathlib import Path
+
+from fastapi import APIRouter
+from fastapi.responses import FileResponse
+from fastapi.staticfiles import StaticFiles
+
+
+def create_frontend_router(frontend_dir: Path) -> APIRouter:
+    """Create a router for serving the frontend SPA."""
+    router = APIRouter(tags=["frontend"])
+
+    router.mount("/assets", StaticFiles(directory=frontend_dir / "assets"), name="assets")
+
+    @router.get("/{full_path:path}")
+    async def serve_spa(full_path: str) -> FileResponse:
+        """Serve React SPA for all non-API routes."""
+        file_path = frontend_dir / full_path
+        if file_path.is_file():
+            return FileResponse(file_path)
+        return FileResponse(frontend_dir / "index.html")
+
+    return router