feat(frontend): scaffold PWA with LiveKit join flow

Vite + React + TS + Tailwind v4, installable PWA. Join-pod UI that fetches a
LiveKit token from the backend, connects to the pod room, and publishes screen
+ mic so PodMan can watch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/index.css

@@ -0,0 +1,16 @@
+@import 'tailwindcss';
+
+:root {
+  color-scheme: dark;
+}
+
+body {
+  margin: 0;
+  background: #0b0f17;
+  color: #e7eaf0;
+  font-family:
+    ui-sans-serif,
+    system-ui,
+    -apple-system,
+    sans-serif;
+}