feat(background): implement user consent mechanism for auto-start functionality

Add auto-start consent system that requires explicit user permission before enabling background sync services. Users can now choose between silent mode (no auto-start) or background mode (15-minute sync intervals).

- Add consent service with AutoStartMode enum and storage utilities
- Create withRemoveAutoStart Expo plugin to disable Android auto-start defaults
- Integrate consent checks into JPush service, background task manager, and foreground service
- Add auto-start consent UI in notification settings with descriptive dialog
- Update privacy policy with section 8 explaining auto-start scenarios and user controls
- Change default sync mode from BATTERY_SAVER to DISABLED
- Export reinitBackgroundService function for re-initializing after consent changes
- Merge OTA Android and iOS workflows into matrix build
- Add release signing config in withSigning plugin for Android
- Expand .dockerignore with native credential and build artifact exclusions

app/_layout.tsx

@@ -146,6 +146,7 @@ function NotificationBootstrap() {
       const { systemNotificationService } = await import('@/services/notification');
       await systemNotificationService.initialize();
       const { initBackgroundService } = await import('@/services/background');
+      // 默认静默模式，不会注册后台任务，不会触发自启动
       await initBackgroundService();
 
       const subscription = AppState.addEventListener('change', (nextAppState) => {