Move the theme toggle from the main header to the settings dialog for a
cleaner UI and better organization. The new theme selector uses a
button group design consistent with the language settings.
Changes:
- Created ThemeSettings component with three options: Light, Dark, System
- Added ThemeSettings to the General tab in settings dialog
- Removed ModeToggle component from main header
- Added theme-related i18n keys for all options
- Theme selection takes effect immediately without requiring save
Design:
- Uses button group style matching LanguageSettings
- Icons for each theme option (Sun, Moon, Monitor)
- Consistent with app's blue theme for active state
- Smooth transitions and hover effects
This change simplifies the main header and consolidates all appearance
settings in one place, improving the overall user experience.
