Centralises all SnackBar calls behind UISnackbars.showError/showSuccess/
showWarning/showInfo, each themed via AppColorExtension. New warning and
info colour tokens added to both themes. Calling any method clears any
visible snackbar before showing the new one.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds a dropdown in the settings page to switch between available themes
(Catppuccin Mocha, Gruvbox Dark). The selected theme is applied immediately
on save and persisted across restarts via SharedPreferences.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
