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/src/mobile/useDevice.ts

@@ -0,0 +1,51 @@
+// src/mobile/useDevice.ts
+// 设备检测：判断当前是否移动端，结果存 localStorage 避免重复检测
+
+const STORAGE_KEY = 'device_mode'
+
+/** UA 关键字匹配移动设备 */
+const MOBILE_UA = /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i
+
+/** 屏幕宽度阈值（含平板竖屏） */
+const MOBILE_WIDTH = 768
+
+/**
+ * 原始检测：综合 UA 和屏幕宽度判断
+ * - UA 命中移动设备关键字 → 手机
+ * - 屏宽 <= 768 → 手机
+ * - 否则 → 桌面
+ */
+function detectRaw(): 'mobile' | 'desktop' {
+  if (typeof navigator === 'undefined') return 'desktop'
+  if (MOBILE_UA.test(navigator.userAgent)) return 'mobile'
+  if (typeof window !== 'undefined' && window.innerWidth <= MOBILE_WIDTH) return 'mobile'
+  return 'desktop'
+}
+
+/** 当前设备模式（读取 localStorage，无则检测并存入） */
+export function getDeviceMode(): 'mobile' | 'desktop' {
+  if (typeof localStorage === 'undefined') return detectRaw()
+  const stored = localStorage.getItem(STORAGE_KEY)
+  if (stored === 'mobile' || stored === 'desktop') return stored
+  const detected = detectRaw()
+  localStorage.setItem(STORAGE_KEY, detected)
+  return detected
+}
+
+/** 是否移动端（路由分流用，同步函数） */
+export function isMobile(): boolean {
+  return getDeviceMode() === 'mobile'
+}
+
+/**
+ * 手动切换设备模式（设置页"切换到桌面版/手机版"用）
+ * 切换后需整页刷新以重新走路由解析
+ */
+export function setDeviceMode(mode: 'mobile' | 'desktop') {
+  localStorage.setItem(STORAGE_KEY, mode)
+}
+
+/** 重置为自动检测（登出时调用，避免下个用户沿用上个用户的偏好） */
+export function resetDeviceMode() {
+  localStorage.removeItem(STORAGE_KEY)
+}