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/tailwind.config.js

@@ -0,0 +1,22 @@
+/** @type {import('tailwindcss').Config} */
+export default {
+  content: ["./index.html", "./src/**/*.{ts,tsx}"],
+  theme: {
+    extend: {
+      colors: {
+        bg: "var(--bg)",
+        surface: "var(--surface)",
+        card: "var(--card)",
+        border: "var(--border)",
+        fg: "var(--fg)",
+        muted: "var(--muted)",
+        accent: "var(--accent)",
+        "accent-fg": "var(--accent-fg)",
+      },
+      fontFamily: {
+        sans: ["Inter", "system-ui", "-apple-system", "Segoe UI", "Roboto", "sans-serif"],
+      },
+    },
+  },
+  plugins: [],
+};