Back up apps (part of #20)

lib/constants/app_constants.dart

@@ -22,12 +22,15 @@ const kZapstoreCommunityPubkey =
 /// Identifier for storing user saved apps
 const kAppBookmarksIdentifier = 'zapstore-bookmarks';
 
+/// Identifier for the encrypted backup of installed apps
+const kInstalledAppsBackupIdentifier = 'zapstore-installed-backup';
+
 /// Event filter for app stacks - must run as schemaFilter so rejected events
 /// are never stored in local SQLite.
 ///
 /// Rejects:
 /// - Private/encrypted stacks (non-empty content, kind 30267 only)
-/// - The user's own saved-apps bookmark stack
+/// - The user's own saved-apps bookmark stack and installed-apps backup
 /// - Stacks with no public App (32267) references
 bool appStackEventFilter(Map<String, dynamic> event) {
   // Guard: only apply to AppStack events (kind 30267).
@@ -44,7 +47,10 @@ bool appStackEventFilter(Map<String, dynamic> event) {
 
   for (final tag in tags) {
     if (tag is! List || tag.isEmpty) continue;
-    if (tag[0] == 'd' && tag.length > 1 && tag[1] == kAppBookmarksIdentifier) {
+    if (tag[0] == 'd' &&
+        tag.length > 1 &&
+        (tag[1] == kAppBookmarksIdentifier ||
+            tag[1] == kInstalledAppsBackupIdentifier)) {
       return false;
     }
   }

lib/screens/profile_screen.dart

@@ -16,6 +16,7 @@ import 'package:purplebase/purplebase.dart';
 import 'package:zapstore/main.dart';
 import 'package:zapstore/services/bookmarks_service.dart';
 import 'package:zapstore/services/package_manager/package_manager.dart';
+import 'package:zapstore/services/secure_storage_service.dart';
 import 'package:zapstore/utils/extensions.dart';
 import 'package:zapstore/utils/nostr_route.dart';
 import 'package:zapstore/widgets/common/profile_identity_row.dart';
@@ -1397,6 +1398,35 @@ class _EmptyState extends StatelessWidget {
   }
 }
 
+class _InstalledAppsBackupToggle extends ConsumerWidget {
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final pubkey = ref.watch(Signer.activePubkeyProvider);
+    if (pubkey == null) return const SizedBox.shrink();
+
+    final backupAsync = ref.watch(installedAppsBackupEnabledProvider);
+    final enabled = backupAsync.valueOrNull ?? false;
+
+    return SwitchListTile(
+      secondary: CircleAvatar(
+        radius: 18,
+        backgroundColor: Theme.of(context).colorScheme.primaryContainer,
+        child: Icon(Icons.backup, color: Theme.of(context).colorScheme.primary),
+      ),
+      title: const Text('Back up installed apps'),
+      subtitle: const Text('Encrypted backup to Nostr relays'),
+      value: enabled,
+      contentPadding: EdgeInsets.zero,
+      onChanged: (value) async {
+        await ref
+            .read(secureStorageServiceProvider)
+            .setInstalledAppsBackupEnabled(value);
+        ref.invalidate(installedAppsBackupEnabledProvider);
+      },
+    );
+  }
+}
+
 class _DataManagementSection extends ConsumerWidget {
   const _DataManagementSection();
 
@@ -1413,6 +1443,8 @@ class _DataManagementSection extends ConsumerWidget {
               style: Theme.of(context).textTheme.titleMedium,
             ),
             const SizedBox(height: 16),
+            _InstalledAppsBackupToggle(),
+            const SizedBox(height: 8),
             ListTile(
               leading: CircleAvatar(
                 radius: 18,

lib/services/secure_storage_service.dart

@@ -154,6 +154,21 @@ class SecureStorageService {
       value: jsonEncode(relays.toList()),
     );
   }
+
+  // =========================================================================
+  // Installed Apps Backup
+  // =========================================================================
+
+  static const _installedAppsBackupKey = 'installed_apps_backup_enabled';
+
+  Future<bool> isInstalledAppsBackupEnabled() async {
+    final value = await _storage.read(key: _installedAppsBackupKey);
+    return value == 'true';
+  }
+
+  Future<void> setInstalledAppsBackupEnabled(bool enabled) async {
+    await _storage.write(key: _installedAppsBackupKey, value: '$enabled');
+  }
 }
 
 /// Persists the AmberSigner pubkey in flutter_secure_storage.
@@ -189,3 +204,13 @@ final hasNwcStringProvider = FutureProvider.autoDispose<bool>((ref) async {
   final secureStorage = ref.watch(secureStorageServiceProvider);
   return secureStorage.hasNWCString();
 });
+
+/// Whether the installed apps backup setting is enabled.
+///
+/// Use `ref.invalidate(installedAppsBackupEnabledProvider)` after toggling
+/// the setting to refresh UI.
+final installedAppsBackupEnabledProvider =
+    FutureProvider.autoDispose<bool>((ref) async {
+  final secureStorage = ref.watch(secureStorageServiceProvider);
+  return secureStorage.isInstalledAppsBackupEnabled();
+});

lib/services/updates_service.dart

@@ -92,6 +92,7 @@ class UpdatePollerNotifier extends StateNotifier<UpdatePollerState> {
 
   final Ref ref;
   Timer? _pollTimer;
+  List<String> _lastBackedUpIds = [];
 
   void _init() {
     ref.listen<AsyncValue<void>>(appInitializationProvider, (prev, next) {
@@ -126,6 +127,7 @@ class UpdatePollerNotifier extends StateNotifier<UpdatePollerState> {
         lastCheckTime: DateTime.now(),
         clearError: true,
       );
+      unawaited(_backupInstalledApps());
     } catch (e) {
       debugPrint('[UpdatePoller] Check failed: $e');
       state = state.copyWith(
@@ -186,6 +188,68 @@ class UpdatePollerNotifier extends StateNotifier<UpdatePollerState> {
     state = state.copyWith(catalogedIds: result.catalogedIds);
   }
 
+  /// Best-effort backup of installed apps as an encrypted private stack.
+  /// Runs after each successful update check. Only publishes when the set
+  /// of cataloged installed apps has changed since the last backup.
+  Future<void> _backupInstalledApps() async {
+    final pubkey = ref.read(Signer.activePubkeyProvider);
+    if (pubkey == null) return;
+
+    final enabled = await ref
+        .read(secureStorageServiceProvider)
+        .isInstalledAppsBackupEnabled();
+    if (!enabled) return;
+
+    final signer = ref.read(Signer.activeSignerProvider)!;
+    final pmNotifier = ref.read(packageManagerProvider.notifier);
+    final installed = ref.read(packageManagerProvider).installed;
+    final platform = pmNotifier.platform;
+    final storage = ref.read(storageNotifierProvider.notifier);
+
+    try {
+      final apps = await storage.query(
+        RequestFilter<App>(
+          tags: {
+            '#d': installed.keys.toSet(),
+            '#f': {platform},
+          },
+        ).toRequest(),
+        source: const LocalSource(),
+        subscriptionPrefix: 'app-backup-resolve',
+      );
+
+      final appIds =
+          apps
+              .map((a) => '${a.event.kind}:${a.event.pubkey}:${a.identifier}')
+              .toList()
+            ..sort();
+
+      if (_listEquals(appIds, _lastBackedUpIds)) return;
+
+      final partialStack = PartialAppStack.withEncryptedApps(
+        name: 'Installed Apps',
+        identifier: kInstalledAppsBackupIdentifier,
+        apps: appIds,
+        platform: platform,
+      );
+
+      final signed = await partialStack.signWith(signer);
+      await storage.save({signed});
+      await storage.publish({signed}, relays: {'AppCatalog', 'social'});
+      _lastBackedUpIds = appIds;
+    } catch (e) {
+      debugPrint('[InstalledAppsBackup] Backup failed: $e');
+    }
+  }
+
+  static bool _listEquals(List<String> a, List<String> b) {
+    if (a.length != b.length) return false;
+    for (var i = 0; i < a.length; i++) {
+      if (a[i] != b[i]) return false;
+    }
+    return true;
+  }
+
   @override
   void dispose() {
     _pollTimer?.cancel();

spec/guidelines/INVARIANTS.md

@@ -41,9 +41,7 @@ If any invariant is violated, the implementation is incorrect.
 
 ## App Stack (kind 30267) Tag Rules
 
-- Encrypted stacks (private collections, e.g. "Saved Apps") MUST NOT include a community `p` tag.
-- Only public stacks may carry a community `p` tag to associate them with a community.
-- The `h` tag is used for relay grouping/routing and may appear on both public and private stacks.
+- Encrypted stacks (private collections, e.g. "Saved Apps") MUST NOT include a community `h` tag.
 
 ## Lifecycle Safety