Add app source and tests

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

lib/Peach/configs/ThemeController.dart

@@ -0,0 +1,23 @@
+import 'package:flutter/material.dart';
+
+/// App-wide theme mode. `main.dart` listens to this and rebuilds MaterialApp;
+/// any screen can set it. Persisting the choice is the ViewModel's job — this
+/// only carries the live value.
+class ThemeController {
+  ThemeController._();
+
+  static final ValueNotifier<ThemeMode> mode = ValueNotifier<ThemeMode>(
+    ThemeMode.dark,
+  );
+
+  /// The stored string ('dark' | 'light' | 'system') for the current mode.
+  static String get name => mode.value.name;
+
+  static void apply(String stored) {
+    mode.value = switch (stored) {
+      'light' => ThemeMode.light,
+      'system' => ThemeMode.system,
+      _ => ThemeMode.dark,
+    };
+  }
+}