do not write default values

mobile/lib/infrastructure/repositories/metadata.repository.dart

@@ -50,6 +50,9 @@ class MetadataRepository extends DriftDatabaseRepository {
 
   Future<void> write<T extends Object, U extends T>(MetadataKey<T> key, U value) async {
     if (_read(key) == value) return;
+    if (value == key.defaultValue) {
+      return delete(key);
+    }
 
     await _db
         .into(_db.metadataEntity)

mobile/lib/utils/migration.dart

@@ -241,6 +241,9 @@ class _StoreMigrator {
   Future<void> complete() async {
     await _db.batch((batch) {
       for (final entry in _cache.entries) {
+        if (entry.value == entry.key.defaultValue) {
+          continue;
+        }
         batch.insert(
           _db.metadataEntity,
           MetadataEntityCompanion(key: Value(entry.key.key), value: Value(entry.key.encode(entry.value))),