Add BackgroundPackageManager and InstalledPackagesSnapshot classes for managing package installations and snapshots in the background. This includes methods for saving and loading installed packages, with error handling for file operations.

lib/services/package_manager/background_package_manager.dart

@@ -0,0 +1,53 @@
+import 'package:models/models.dart';
+import 'package:zapstore/services/package_manager/installed_packages_snapshot.dart';
+import 'package:zapstore/services/package_manager/package_manager.dart';
+
+/// Background-safe PackageManager that avoids EventChannel usage.
+final class BackgroundPackageManager extends PackageManager {
+  BackgroundPackageManager(super.ref);
+
+  // Zapstore currently targets arm64 APKs for background checks.
+  @override
+  String get platform => 'android-arm64-v8a';
+
+  @override
+  String get packageExtension => '.apk';
+
+  @override
+  bool get supportsSilentInstall => false;
+
+  @override
+  Future<void> install(
+    String appId,
+    String filePath, {
+    required String expectedHash,
+    required int expectedSize,
+    required FileMetadata target,
+  }) {
+    throw UnsupportedError('Install not supported in background');
+  }
+
+  @override
+  Future<void> uninstall(String appId) {
+    throw UnsupportedError('Uninstall not supported in background');
+  }
+
+  @override
+  Future<void> launchApp(String appId) {
+    throw UnsupportedError('Launch not supported in background');
+  }
+
+  @override
+  Future<void> requestPermission() {
+    throw UnsupportedError('Permission not supported in background');
+  }
+
+  @override
+  Future<bool> hasPermission() async => false;
+
+  @override
+  Future<void> syncInstalledPackages() async {
+    final installed = await InstalledPackagesSnapshot.load();
+    state = state.copyWith(installed: installed);
+  }
+}

lib/services/package_manager/installed_packages_snapshot.dart

@@ -0,0 +1,90 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:path/path.dart' as path;
+import 'package:path_provider/path_provider.dart';
+import 'package:zapstore/services/package_manager/package_manager.dart';
+
+class InstalledPackagesSnapshot {
+  static const _fileName = 'installed_packages_snapshot.json';
+
+  static Future<File> _file() async {
+    final dir = await getApplicationSupportDirectory();
+    return File(path.join(dir.path, _fileName));
+  }
+
+  static Future<void> save(Map<String, PackageInfo> installed) async {
+    try {
+      final file = await _file();
+      final tmp = File('${file.path}.tmp');
+      final list = installed.values
+          .map(
+            (p) => <String, dynamic>{
+              'appId': p.appId,
+              'name': p.name,
+              'version': p.version,
+              'versionCode': p.versionCode,
+              'signatureHash': p.signatureHash,
+              'canInstallSilently': p.canInstallSilently,
+            },
+          )
+          .toList(growable: false);
+      final payload = jsonEncode({
+        'v': 1,
+        'savedAt': DateTime.now().millisecondsSinceEpoch,
+        'installed': list,
+      });
+      await tmp.writeAsString(payload, flush: true);
+      if (await file.exists()) {
+        await file.delete();
+      }
+      await tmp.rename(file.path);
+    } catch (e) {
+      // Best-effort snapshot only.
+      if (kDebugMode) {
+        debugPrint('[InstalledPackagesSnapshot] Save failed: $e');
+      }
+    }
+  }
+
+  static Future<Map<String, PackageInfo>> load() async {
+    try {
+      final file = await _file();
+      if (!await file.exists()) return {};
+
+      final raw = await file.readAsString();
+      final decoded = jsonDecode(raw);
+      if (decoded is! Map) return {};
+      final version = decoded['v'];
+      if (version != null && version is! int && kDebugMode) {
+        debugPrint('[InstalledPackagesSnapshot] Unknown schema: $version');
+      }
+      final installed = decoded['installed'];
+      if (installed is! List) return {};
+
+      final result = <String, PackageInfo>{};
+      for (final item in installed) {
+        if (item is! Map) continue;
+        final map = Map<String, dynamic>.from(item);
+        final appId = map['appId'] as String?;
+        if (appId == null || appId.isEmpty) continue;
+        result[appId] = PackageInfo(
+          appId: appId,
+          name: map['name'] as String?,
+          version: (map['version'] as String?) ?? '0.0.0',
+          versionCode: map['versionCode'] as int?,
+          signatureHash: (map['signatureHash'] as String?) ?? '',
+          installTime: null,
+          canInstallSilently: (map['canInstallSilently'] as bool?) ?? false,
+        );
+      }
+      return result;
+    } catch (e) {
+      if (kDebugMode) {
+        debugPrint('[InstalledPackagesSnapshot] Load failed: $e');
+      }
+      return {};
+    }
+  }
+}