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/src/components/Login.tsx

@@ -0,0 +1,23 @@
+export default function Login() {
+  return (
+    <div className="min-h-screen grid place-items-center bg-bg text-fg">
+      <div className="w-[min(92vw,420px)] rounded-2xl border border-border bg-card/70 backdrop-blur p-10 text-center shadow-2xl">
+        <div className="text-3xl font-bold tracking-tight">
+          Sub<span className="text-accent">feed</span>
+        </div>
+        <p className="text-muted my-5 leading-relaxed">
+          Your subscriptions, filtered your way.
+          <br />
+          Sign in with your Google account.
+        </p>
+        <a
+          href="/auth/login"
+          className="inline-flex items-center gap-2 px-5 py-3 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+        >
+          Sign in with Google
+        </a>
+        <div className="text-xs text-muted mt-6">Invite-only access.</div>
+      </div>
+    </div>
+  );
+}