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/pages/home_page.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../app/locale_controller.dart';
 import '../core/api_client.dart';
+import '../core/network_status.dart';
 import '../features/albums/providers.dart';
 import '../features/uploads/engine_scope.dart';
 import '../features/uploads/queue_controller.dart';
@@ -38,10 +39,48 @@ class _HomePageState extends ConsumerState<HomePage> {
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
     final badgeCount = ref.watch(uploadBadgeCountProvider);
+    // P0-4：离线黄条（网络恢复自动消失）。
+    final network = ref.watch(networkStatusProvider).value;
+    final offline = network == NetworkAvailability.offline;
     return Scaffold(
-      body: IndexedStack(
-        index: _selectedIndex,
-        children: const [AlbumsTab(), QueueTab(), SettingsTab()],
+      body: Column(
+        children: [
+          if (offline)
+            Material(
+              color: Theme.of(context).colorScheme.tertiaryContainer,
+              child: SizedBox(
+                width: double.infinity,
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+                  child: Row(
+                    children: [
+                      Icon(
+                        Icons.wifi_off_rounded,
+                        size: 16,
+                        color: Theme.of(context).colorScheme.onTertiaryContainer,
+                      ),
+                      const SizedBox(width: 8),
+                      Expanded(
+                        child: Text(
+                          l10n.networkOfflineBanner,
+                          style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                                color:
+                                    Theme.of(context).colorScheme.onTertiaryContainer,
+                              ),
+                        ),
+                      ),
+                    ],
+                  ),
+                ),
+              ),
+            ),
+          Expanded(
+            child: IndexedStack(
+              index: _selectedIndex,
+              children: const [AlbumsTab(), QueueTab(), SettingsTab()],
+            ),
+          ),
+        ],
       ),
       bottomNavigationBar: NavigationBar(
         selectedIndex: _selectedIndex,