Fix sync file structure and implement iOS BGTaskScheduler

[Copilot]

NativeScript's module resolution prioritizes sync.${platform}.ts over sync.ts, causing the common sync logic to be skipped entirely on both platforms.

Changes

File Structure

• sync.ts → sync.common.ts - shared SyncService implementation
• sync.android.ts - exports from common, sets Android AlarmManager implementations
• sync.ios.ts - exports from common, sets iOS BGTaskScheduler implementations

Platform files inject their implementations into the common module:

// sync.android.ts
export * from './sync.common';
import * as syncCommon from './sync.common';

syncCommon.scheduleAndroidSyncAlarm = (serviceId, delayMs) => {
  // Android AlarmManager implementation
};

When importing ~/services/sync, NativeScript loads the platform-specific file which re-exports everything from common plus platform implementations.

iOS Background Sync

• Registers BGTaskScheduler handler with task identifier ${__APP_ID__}.sync
• Maintains Map<serviceId, scheduledTime> for pending syncs across all services
• Schedules single background task for earliest required sync time
• Processes all pending syncs concurrently when task executes
• Falls back to setTimeout on iOS < 13

Android

• Unchanged behavior, now injected into common module instead of dynamically imported

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