feat: improve Extension Store with custom icons and various fixes

- Support custom extension icons from registry (iconUrl field)
- Support both camelCase and snake_case in registry JSON
- Fix download file extension (.spotiflac-ext)
- New extensions start disabled by default
- Preserve enabled state on extension upgrade
- Add toggle to show/hide Store tab in Settings > Options
- Reorder tabs: Home, History, Store, Settings

lib/providers/settings_provider.dart

@@ -216,6 +216,11 @@ class SettingsNotifier extends Notifier<AppSettings> {
     state = state.copyWith(separateSingles: enabled);
     _saveSettings();
   }
+
+  void setShowExtensionStore(bool enabled) {
+    state = state.copyWith(showExtensionStore: enabled);
+    _saveSettings();
+  }
 }
 
 final settingsProvider = NotifierProvider<SettingsNotifier, AppSettings>(