feat: M4 (part 2) — React reader UI with theming

- Vite + React + TS + Tailwind SPA served by FastAPI (multi-stage Docker build)
- Four color schemes (Midnight default, Forest, Slate, YouTube) x dark/light,
  adjustable text size; persisted to user preferences and localStorage
- Header search, grid/list toggle, theme menu; sidebar filters (show state,
  sort, include Shorts/live, language + topic tag chips with any/all matching)
- Infinite-scroll feed of video cards; click opens youtube.com in a new tab;
  per-video watched / saved / hidden actions with optimistic updates
- SPA fallback routing; login screen for unauthenticated users

backend/app/main.py

@@ -1,7 +1,7 @@
 from contextlib import asynccontextmanager
 from pathlib import Path
 
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
@@ -47,10 +47,23 @@ app.include_router(tags.router)
 app.include_router(feed.router)
 app.include_router(me.router)
 
-STATIC_DIR = Path(__file__).parent / "static"
-app.mount("/assets", StaticFiles(directory=STATIC_DIR / "assets"), name="assets")
+# The built SPA (populated by the Docker frontend build stage).
+STATIC_DIR = Path(__file__).parent / "static_spa"
+app.mount(
+    "/assets",
+    StaticFiles(directory=STATIC_DIR / "assets", check_dir=False),
+    name="assets",
+)
 
 
 @app.get("/")
 async def index() -> FileResponse:
     return FileResponse(STATIC_DIR / "index.html")
+
+
+@app.get("/{full_path:path}")
+async def spa_fallback(full_path: str) -> FileResponse:
+    # Client-side routes fall back to index.html; real API/asset paths are matched above.
+    if full_path.startswith(("api/", "auth/", "healthz", "assets/")):
+        raise HTTPException(status_code=404)
+    return FileResponse(STATIC_DIR / "index.html")