feat(mobile): flutter scaffold with env script and baseline nav

- scripts/flutter-env.sh: toolchain PATH/JAVA_HOME/ANDROID_HOME + pub 镜像 env
- apps/mobile: flutter create (com.photolive.photo_live, android+ios)
  - Android minSdk 24 / targetSdk 36；INTERNET 权限（main manifest，debug+release）
  - iOS Info.plist：本地网络 ATS 例外 + 相册/相机用途说明（zh+en）
- 依赖接入：riverpod / drift(+sqlite3_flutter_libs) / dio / secure_storage /
  image_picker / camera / qr_flutter / path_provider / path / intl
- l10n：l10n.yaml + app_zh/app_en arb（20 键），gen-l10n（zh 模板，非空 getter）
- 基线：Material 3 主题、/login→/home 命名路由、底部导航（相册/队列/设置桩）、
  locale provider 脚手架（Task 10 完成切换）
- flutter analyze 零问题；widget 测试通过；debug apk 构建成功
  （MuMu 冒烟因宿主 commit 内存耗尽未完成，Task 2 重试）

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

apps/mobile/lib/l10n/app_en.arb

@@ -0,0 +1,24 @@
+{
+  "@@locale": "en",
+  "appName": "Photo Live",
+  "login": "Sign in",
+  "logout": "Sign out",
+  "username": "Account",
+  "password": "Password",
+  "albums": "Albums",
+  "albumDetail": "Album details",
+  "queue": "Upload queue",
+  "settings": "Settings",
+  "language": "Language",
+  "localeFollowSystem": "Follow system",
+  "localeChinese": "中文",
+  "localeEnglish": "English",
+  "retry": "Retry",
+  "cancel": "Cancel",
+  "errorGeneric": "Something went wrong, please retry",
+  "ok": "OK",
+  "homeTabAlbums": "Albums",
+  "homeTabQueue": "Queue",
+  "homeTabSettings": "Settings",
+  "loginHint": "Sign in with your staff account"
+}