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
     />
   )
 }

frontend/src/index.css

@@ -222,4 +222,20 @@
   .list-item-interactive:active {
     background-color: hsl(var(--elevated));
   }
+
+  /* Panel empty state — centered muted text */
+  .panel-empty {
+    text-align: center;
+    padding: 1.25rem 0.625rem;
+    color: hsl(var(--text-muted));
+    font-size: 0.5625rem; /* 9px */
+  }
+
+  /* Panel error state — centered error text */
+  .panel-error {
+    text-align: center;
+    padding: 1.25rem 0.625rem;
+    color: hsl(var(--error));
+    font-size: 0.5625rem;
+  }
 }