feat(mobile): stage 1 - infrastructure (vant, useDevice, MobileLayout, router, mobile.css)

- add vant@^4.9.0 dependency
- migrate useDevice.ts (device detection + localStorage)
- migrate MobileLayout.vue (tabbar + navbar + store init)
- migrate mobile.css (vant theme -> --gg-* mapping)
- migrate Placeholder.vue for unimplemented mobile views
- index.html: viewport for mobile (no-zoom, safe-area)
- main.ts: register Vant + import mobile.css
- vite.config.ts: manualChunks code splitting (vue/element/vant/pocketbase/livekit)
- router/index.ts: device-based routing via isMobile() + view() helper,
  preserve uat's JoinGroup/JoinTeam routes, mobile views use Placeholder
  pending stages 2-11

build verified: vue-tsc + vite build pass

frontend/vite.config.ts

@@ -9,6 +9,25 @@ export default defineConfig({
       '@': path.resolve(__dirname, 'src')
     }
   },
+  build: {
+    rollupOptions: {
+      output: {
+        // 代码分割：vendor 按依赖分组，避免单个超大 chunk
+        manualChunks: {
+          // Vue 核心运行时（vue + vue-router + pinia）
+          'vue-vendor': ['vue', 'vue-router', 'pinia'],
+          // 桌面端 UI 库
+          'element-plus': ['element-plus', '@element-plus/icons-vue'],
+          // 手机端 UI 库
+          'vant': ['vant'],
+          // 后端 SDK
+          'pocketbase': ['pocketbase'],
+          // 语音房依赖（仅 VoiceRoom 用到，体积大，单独拆分）
+          'livekit': ['livekit-client'],
+        }
+      }
+    }
+  },
   server: {
     port: Number(process.env.VITE_PORT) || 5173,
     proxy: {