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

docker-compose.yml

@@ -16,7 +16,8 @@ services:
 
   api:
     build:
-      context: ./backend
+      context: .
+      dockerfile: Dockerfile
     env_file: .env
     environment:
       DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-subfeed}:${POSTGRES_PASSWORD:-subfeed}@db:5432/${POSTGRES_DB:-subfeed}