feat(ui): close 3 P2 audit findings + a11y sweep (Sprint 3)

- lib/toast.tsx (renamed from .ts for JSX): new showToast.undo(message,
  onUndo, ms=5000) helper. Inline 'Undo' button dismisses the toast and
  fires onUndo. Note: react-hot-toast 2.6 lacks onClose/onDismiss, so
  expiry is silent — same effective behavior as confirm() declined.

- Dashboard.handleDelete: captures the full MealPlanItem before the
  DELETE so Undo can re-fire meals.generateItem(planId, dayOfWeek,
  mealType) and refill the slot (recipe may differ — see plan R4).

- Pantry.handleRemove: fully reversible — Undo re-fires pantry.add with
  the original ingredient_id, quantity, and unit. New removeId state
  scopes the spinner to the clicked row.

- Both confirm() call sites removed.

- App.tsx Navigation: whitespace-nowrap + px-2 sm:px-3 so all 4 links fit
  on one line down to 360 px. aria-current='page' on the active link.
  <nav aria-label='Primary'>, <main id='main-content'>.

- components/ui/Badge: optional icon and aria-label props. Dashboard
  approval-status Badge passes aria-label='Approval status: approved'
  (or the current value) so screen readers don't rely on color alone.

- ErrorBoundary already mounted at App.tsx:42 — verified, no code change.

- Review/sprint3-verification.md (new) + Review/ui-nielsen-audit.md and
  fix-ui-audit.md updated with Sprint 3 status and deploy steps.

Build: npm run build (tsc + vite) green. tsc 0 errors.

frontend/src/lib/toast.tsx

@@ -0,0 +1,41 @@
+import toast from 'react-hot-toast';
+
+export const showToast = {
+  success: (message: string) => toast.success(message),
+  error: (message: string) => toast.error(message),
+  loading: (message: string) => toast.loading(message),
+  dismiss: (toastId: string) => toast.dismiss(toastId),
+  promise: <T,>(
+    promise: Promise<T>,
+    messages: { loading: string; success: string; error: string }
+  ) => toast.promise(promise, messages),
+  /**
+   * Show a confirmation toast with an Undo button. The toast stays
+   * visible for `ms` (default 5000) and the user can click Undo to
+   * fire `onUndo`. If the user does nothing, the toast just dismisses.
+   */
+  undo: (message: string, onUndo: () => void, ms = 5000) => {
+    return toast(
+      (t) => (
+        <span className="flex items-center gap-3">
+          <span>{message}</span>
+          <button
+            type="button"
+            onClick={() => {
+              onUndo();
+              toast.dismiss(t.id);
+            }}
+            className="font-semibold text-primary-600 hover:text-primary-700 focus:outline-none focus:ring-2 focus:ring-primary-400 rounded px-1"
+          >
+            Undo
+          </button>
+        </span>
+      ),
+      {
+        duration: ms,
+        icon: '\u2715',
+        style: { background: '#fff', color: '#404040' },
+      }
+    );
+  },
+};