Extract snackbar notifications to UISnackbars utility

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>

frontend/lib/theme/gruvbox_theme.dart

@@ -81,6 +81,10 @@ final ThemeData gruvboxDarkTheme = ThemeData(
     const AppColorExtension(
       success: GruvboxColors.brightGreen,
       onSuccess: GruvboxColors.bgHard,
+      warning: GruvboxColors.brightYellow,
+      onWarning: GruvboxColors.bgHard,
+      info: GruvboxColors.brightBlue,
+      onInfo: GruvboxColors.bgHard,
     ),
   ],
 );