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/vite.config.ts

@@ -0,0 +1,15 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+// During `vite dev` (host with Node), proxy API calls to the backend container.
+const proxy = {
+  "/api": "http://localhost:8080",
+  "/auth": "http://localhost:8080",
+  "/healthz": "http://localhost:8080",
+};
+
+export default defineConfig({
+  plugins: [react()],
+  server: { port: 5173, proxy },
+  build: { outDir: "dist" },
+});