fix(mob): stop the local database from becoming unreadable

mobile/lib/controllers/chat/inbox_controller.dart

@@ -14,12 +14,16 @@ class InboxController extends ChangeNotifier {
   bool isLoading = false;
   bool hasLoadedOnce = false;
 
+  /// Why the last load failed, shown instead of an empty inbox.
+  String? loadError;
+
   List<InboxItem> get groups => inbox.where((item) => item.isGroup).toList();
   int get unreadChats => inbox.where((item) => item.unreadCount > 0).length;
 
   void clearInbox() {
     inbox = [];
     hasLoadedOnce = false;
+    loadError = null;
     notifyListeners();
   }
 
@@ -36,6 +40,7 @@ class InboxController extends ChangeNotifier {
       }
     } catch (e) {
       debugPrint("Failed to load local inbox cache: $e");
+      loadError = "Couldn't open your saved conversations.";
     }
 
     if (isOffline) return;
@@ -88,6 +93,7 @@ class InboxController extends ChangeNotifier {
 
       inbox = combined;
       hasLoadedOnce = true;
+      loadError = null;
       await _replaceInboxInDb(inbox);
 
       for (final chat in chatsToCatchUp) {
@@ -97,6 +103,10 @@ class InboxController extends ChangeNotifier {
       }
     } catch (e) {
       debugPrint("Inbox fetch failed: $e");
+      loadError = ApiService.errorMessage(
+        e,
+        fallback: "Couldn't load your conversations.",
+      );
     } finally {
       isLoading = false;
       notifyListeners();

mobile/lib/pages/home/home_page.dart

@@ -308,6 +308,14 @@ class _ChatsTabState extends State<_ChatsTab> {
                       title: 'No matches',
                       message: 'No chat matches "$_query".',
                     )
+                  : inbox.loadError != null
+                  ? EmptyState(
+                      icon: Icons.cloud_off_rounded,
+                      title: 'Couldn\'t load chats',
+                      message: inbox.loadError!,
+                      actionLabel: 'Try again',
+                      onAction: () => inbox.loadInbox(),
+                    )
                   : EmptyState(
                       icon: Icons.forum_rounded,
                       title: 'No conversations yet',

mobile/lib/services/db_services.dart

@@ -10,16 +10,24 @@ import '../core/message_envelope.dart';
 class DatabaseHelper {
   static final DatabaseHelper instance = DatabaseHelper._init();
   static Database? _database;
+  static Future<Database>? _opening;
   final FlutterSecureStorage _secureStorage = const FlutterSecureStorage();
 
   static const String _dbName = 'secure_chat.db';
 
   DatabaseHelper._init();
 
+  /// Opens the database once. Concurrent first callers share the same
+  /// future, so the encryption key is only ever generated once.
   Future<Database> get database async {
-    if (_database != null) return _database!;
-    _database = await _initDB(_dbName);
-    return _database!;
+    final db = _database;
+    if (db != null) return db;
+    final opening = _opening ??= _initDB(_dbName);
+    try {
+      return _database = await opening;
+    } finally {
+      _opening = null;
+    }
   }
 
   Future<String> _getEncryptionKey() async {
@@ -63,7 +71,22 @@ class DatabaseHelper {
     final path = join(dbPath, filePath);
     final password = await _getEncryptionKey();
 
-    return await openDatabase(
+    try {
+      return await _open(path, password);
+    } on DatabaseException catch (e) {
+      if (!await databaseExists(path)) rethrow;
+      // The file was encrypted with a key we no longer have. It only holds a
+      // cache and this device's Signal keys, which are regenerated and
+      // republished on the next sign-in, so start over instead of failing on
+      // every launch.
+      debugPrint('Local database unreadable, recreating it: $e');
+      await deleteDatabase(path);
+      return await _open(path, password);
+    }
+  }
+
+  Future<Database> _open(String path, String password) {
+    return openDatabase(
       path,
       version: 2,
       password: password,