fix(theme): listen to platformDispatcher.onPlatformBrightnessChanged for ThemeMode.system

[ZhuchkaTriplesix]

Summary

• Register WidgetsBindingObserver inside ThemeController when _themeMode == ThemeMode.system to observe OS brightness changes.
• Implement didChangePlatformBrightness to invoke _notifyThemeChanged() when the platform brightness actually changes, cleanly triggering UI theme recomputation.
• Ensure the observer is automatically unregistered when switching to ThemeMode.light or ThemeMode.dark, on resetToDefaults(), and on dispose().
• Add unit tests verifying dynamic brightness transitions and observer cleanup.

Closes #899