fix(spa): serve index.html for SvelteKit client-side routes

Deep-linking to non-root URLs like /settings or /notification-trackers
returned {"detail":"Not Found"} because StaticFiles(html=True) only
serves index.html for directory roots, not for arbitrary SPA routes.

Subclass StaticFiles with an SPA fallback: any 404 on a non-/api path
serves the root index.html, letting the SvelteKit router hydrate the
correct view on the client. Real /api/* 404s still bubble up as JSON
from FastAPI.

packages/server/src/notify_bridge_server/main.py

@@ -158,7 +158,27 @@ async def health():
 from pathlib import Path
 if _cfg.static_dir and Path(_cfg.static_dir).is_dir():
     from fastapi.staticfiles import StaticFiles
-    app.mount("/", StaticFiles(directory=_cfg.static_dir, html=True), name="frontend")
+    from starlette.responses import FileResponse
+    from starlette.exceptions import HTTPException as StarletteHTTPException
+
+    _static_dir = Path(_cfg.static_dir)
+
+    class SPAStaticFiles(StaticFiles):
+        """StaticFiles that falls back to index.html for SvelteKit client-side routes.
+
+        Unknown paths return index.html so that deep links like /settings
+        hydrate the SPA, while /api/* and real asset 404s behave normally.
+        """
+
+        async def get_response(self, path: str, scope):
+            try:
+                return await super().get_response(path, scope)
+            except StarletteHTTPException as exc:
+                if exc.status_code == 404 and not path.startswith("api/"):
+                    return FileResponse(_static_dir / "index.html")
+                raise
+
+    app.mount("/", SPAStaticFiles(directory=_cfg.static_dir, html=True), name="frontend")
 
 
 def run():