fix: resolve import backup silently failing due to deactivated widget context

[ivanwongtf]

Summary

Fixes #111

Import Backup in settings silently fails because the confirmation dialog's pop deactivates the parent widget context before _import() can access service providers.

Problem

In _confirmFileImport(), after the user clicks Import:

1. Navigator.of(dialogContext).pop() closes the confirmation dialog
2. _import(context, ...) is called with the now-deactivated parent context
3. context.read<BackupService>() throws "Looking up a deactivated widget's ancestor is unsafe"
4. Since _import() is called without await, the error becomes an unhandled async exception and is silently swallowed

Fix

Read service providers before showing the dialog while the context is still valid:

Future<void> _confirmFileImport(
    BuildContext context, AppLocalizations localizations, BackupFileEntry entry) async {
  // Read services BEFORE showing dialog while context is still valid
  final backupService = context.read<BackupService>();
  final settingsService = context.read<SettingsService>();
  final appsService = context.read<AppsService>();
  
  showDialog(
    context: context,
    builder: (dialogContext) => AlertDialog(
      // ... dialog with Import button that calls:
      // backupService.importBackup(entry.file)
      // instead of relying on deactivated context
    ),
  );
}

Changes

• lib/widgets/settings/backup_restore_page.dart: Read services before dialog, call importBackup() directly
• Removed the _import() helper method that was the source of the deactivated context bug

Testing

Verified on Samsung SM-F946U (Android 16) with 188 apps across 13 categories - import now works correctly.