refactor(mobile): add Isar DB & Store class (#1574)

* refactor(mobile): add Isar DB & Store class

new Store: globally accessible key-value store like Hive (but based on Isar)

replace first few places of Hive usage with the new Store

* reduce max. DB size to prevent errors on older iOS devices

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Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/lib/shared/providers/db.provider.dart

@@ -0,0 +1,5 @@
+import 'package:hooks_riverpod/hooks_riverpod.dart';
+import 'package:isar/isar.dart';
+
+// overwritten in main.dart due to async loading
+final dbProvider = Provider<Isar>((_) => throw UnimplementedError());