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/vite.config.ts

@@ -0,0 +1,33 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+import { VitePWA } from 'vite-plugin-pwa'
+
+export default defineConfig({
+  plugins: [
+    react(),
+    VitePWA({
+      registerType: 'autoUpdate',
+      manifest: {
+        name: 'Elec',
+        short_name: 'Elec',
+        description: '多模型AI对话',
+        theme_color: '#000000',
+        background_color: '#000000',
+        display: 'standalone',
+        orientation: 'portrait',
+        icons: [
+          {
+            src: '/icon-192.png',
+            sizes: '192x192',
+            type: 'image/png'
+          },
+          {
+            src: '/icon-512.png',
+            sizes: '512x512',
+            type: 'image/png'
+          }
+        ]
+      }
+    })
+  ]
+})