Code Cleanup and Technical Debt Reduction

[jsattler]

Perform a general cleanup of the codebase to improve maintainability.

Areas to improve:

• Magic Strings: SettingsStore uses hardcoded string keys for UserDefaults. Move these to a private enum Keys.
• ViewModel Responsibilities: RecorderViewModel handles permissions, notifications, and timer logic. Consider extracting a NotificationService and TimerService.
• Magic Numbers: RecorderViewModel.getContentSize has hardcoded fallback resolution (1920x1080).
• Error Handling: Ensure AssetWriter errors are propagated clearly to the UI.