Simplify theme management, integrate ToggleButtons for theme selection, and enhance settings view structure.

finlog_app/app/lib/core/app/theme.dart

@@ -25,10 +25,10 @@ class ThemeController extends ChangeNotifier {
   }
 
   /// Sets theme and persists it in Preferences.
-  Future<void> setTheme(ThemeMode mode) async {
+  void setTheme(ThemeMode mode) {
     _themeMode = mode;
     notifyListeners();
-    await _prefs.setString('theme', _toString(mode));
+    _prefs.setString('theme', mode.name);
   }
 
   ThemeMode _fromString(String value) {
@@ -43,16 +43,5 @@ class ThemeController extends ChangeNotifier {
     }
   }
 
-  String _toString(ThemeMode mode) {
-    switch (mode) {
-      case ThemeMode.light:
-        return 'light';
-      case ThemeMode.dark:
-        return 'dark';
-      case ThemeMode.system:
-        return 'system';
-    }
-  }
-
   ThemeMode get themeMode => _themeMode;
 }