docs(storybook): clarify global decorator guarantees nav/auth context (Phase 3)

All stories run under AuthProvider and MemoryRouter; no story should
crash on missing useContext for auth or router.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/web/.storybook/decorators.tsx

@@ -2,6 +2,9 @@
  * Global Storybook decorator: single point of entry for all app providers.
  * No story should import or wrap with these providers directly; they are applied here.
  * Stories that need a specific route can set parameters.router.initialEntries.
+ *
+ * This ensures useAuth, useNavigate, useSearchParams, useQueryClient, useToast
+ * and similar hooks never run without context (no "must be used within XProvider" crashes).
  */
 import React from 'react';
 import type { Decorator } from '@storybook/react';