feat(ui): add error/empty state classes, update toast styling

- Toast: bg-elevated surface, border-default, radius-control, remove
  richColors (use token colors instead of Sonner defaults)
- Add .panel-empty and .panel-error CSS utility classes
- Available for Phase 4 page-level refinement

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/toast.tsx

@@ -8,11 +8,10 @@ export function Toaster() {
     <SonnerToaster
       position="bottom-right"
       toastOptions={{
-        className: 'bg-panel border-border text-text-primary',
+        className: 'bg-elevated border border-border-default text-text-primary rounded-[var(--radius-control)]',
         descriptionClassName: 'text-text-secondary',
       }}
       theme={theme}
-      richColors
     />
   )
 }