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/main.tsx

@@ -0,0 +1,17 @@
+import React from "react";
+import { createRoot } from "react-dom/client";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import App from "./App";
+import "./index.css";
+
+const queryClient = new QueryClient({
+  defaultOptions: { queries: { retry: false, staleTime: 30_000, refetchOnWindowFocus: false } },
+});
+
+createRoot(document.getElementById("root")!).render(
+  <React.StrictMode>
+    <QueryClientProvider client={queryClient}>
+      <App />
+    </QueryClientProvider>
+  </React.StrictMode>
+);