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

frontend/package.json

@@ -0,0 +1,27 @@
+{
+  "name": "subfeed-frontend",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@tanstack/react-query": "^5.51.0",
+    "clsx": "^2.1.1",
+    "lucide-react": "^0.408.0",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1"
+  },
+  "devDependencies": {
+    "@types/react": "^18.3.3",
+    "@types/react-dom": "^18.3.0",
+    "@vitejs/plugin-react": "^4.3.1",
+    "autoprefixer": "^10.4.19",
+    "postcss": "^8.4.39",
+    "tailwindcss": "^3.4.6",
+    "typescript": "^5.5.3",
+    "vite": "^5.3.4"
+  }
+}