feat(app): P0 batch 3 — offline detection with engine pause/resume + UI banner

P0-4 离线感知：
  - connectivity_plus → NetworkAvailability（online/offline/unknown）
  - engine_scope 三重绑定：有账号+有网 → start；任一失去 → stop（在途不中断）
  - 网络恢复 → start + onNetworkRestored（waitingNetwork 即时重试）
  - HomePage 顶部离线黄条（wifi_off + i18n 双语）

P0-5 更正：EXIF 方向已由服务端 sharp .rotate() 自动转正——v2 分析误报

397/397 全绿、analyze 0

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

apps/mobile/lib/l10n/app_localizations_en.dart

@@ -1034,4 +1034,8 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get albumUploadDisabled => 'Upload is disabled for this album';
+
+  @override
+  String get networkOfflineBanner =>
+      'No network. Photos will upload automatically when reconnected.';
 }