feat: add PWA — mobile multi-model chat app (Vite + React)

- Vite + React + TypeScript + Tailwind v4
- vite-plugin-pwa for service worker and manifest
- Dark mode, borderless design with glow effects
- Reuses Elec components, adapted for mobile viewport
- Standalone-ready: can install to phone home screen

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

PWA/postcss.config.js

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  }
+}