feat(ui): global keyboard shortcuts + shortcut help banner (Sprint 5 F2)

F2 — Vim-style keyboard shortcuts (the audit's F2 / H7 finding).

New files:
- frontend/src/hooks/useKeyboardShortcuts.ts: lightweight global
  handler. Supports both single keys ('/', '?', 'Escape') and
  vim-style 2-key sequences ('g d', 'g r', 'g p', 'g s' for nav).
  Sequence timeout is 1500ms; pending prefix is cleared on any
  unrecognised key so typing 'g' alone is safe. Suppressed when
  the user is typing in an input/textarea/select/contenteditable,
  or when any modifier key (Ctrl/Cmd/Alt) is held — those chords
  belong to the browser or other handlers. Uses a ref so the
  listener is registered once and always sees the latest callbacks.

- frontend/src/hooks/useFocusSearch.ts: tiny CustomEvent bus.
  requestFocusSearch() dispatches a 'mealplanner:focus-search'
  event; useFocusSearchOnShortcut(ref) subscribes and focuses the
  supplied input. The decoupling lets any page opt in without the
  global handler needing to know the page's DOM.

- frontend/src/components/ShortcutHelpBanner.tsx: dismissible help
  dialog that slides down under the nav when '?' is pressed.
  Auto-dismisses after 6s; Escape also dismisses. role=dialog +
  aria-label for screen readers; the kbd elements use the
  <kbd> semantic for assistive tech.

Wired in App.tsx:
- New <GlobalShortcuts /> child of <BrowserRouter> calls
  useKeyboardShortcuts with the 4 nav sequences, '/' →
  requestFocusSearch(), and '?' → dispatch SHOW_SHORTCUT_HELP_EVENT.
- <ShortcutHelpBanner /> mounted inside the page wrapper (after
  <main>).

Pantry and Recipes now call useFocusSearchOnShortcut with a
forwardRef attached to their top search inputs. Recipes's search
already debounced via handleSearch so focusing just selects the
existing text for the user to replace. Pantry's search is a plain
controlled input, same treatment.

Behaviour summary:
- g d / g r / g p / g s → navigate to the 4 main pages
- / → focus the search input on the current page (Pantry + Recipes
  only — other pages have no search)
- ? → show the help banner
- All shortcuts are no-ops inside text-entry controls, so a user
  typing 'p' into the pantry search box will not trigger navigation.

Build: tsc 0 errors, vite 0 errors. 5 files, +185/-3.

frontend/src/App.tsx

@@ -1,7 +1,10 @@
-import { BrowserRouter, Routes, Route, Link, useLocation, Navigate } from 'react-router-dom'
+import { BrowserRouter, Routes, Route, Link, useLocation, useNavigate, Navigate } from 'react-router-dom'
 import { QueryClient, QueryClientProvider, QueryCache, MutationCache } from '@tanstack/react-query'
 import { ErrorBoundary } from './components/ErrorBoundary'
+import { ShortcutHelpBanner, SHOW_SHORTCUT_HELP_EVENT } from './components/ShortcutHelpBanner'
 import { showApiError } from './lib/toast'
+import { useKeyboardShortcuts } from './hooks/useKeyboardShortcuts'
+import { requestFocusSearch } from './hooks/useFocusSearch'
 import Dashboard from './pages/Dashboard'
 import MealDetail from './pages/MealDetail'
 import Pantry from './pages/Pantry'
@@ -56,11 +59,25 @@ function Navigation() {
   )
 }
 
+function GlobalShortcuts() {
+  const navigate = useNavigate()
+  useKeyboardShortcuts({
+    'g d': () => navigate('/'),
+    'g r': () => navigate('/recipes'),
+    'g p': () => navigate('/pantry'),
+    'g s': () => navigate('/shopping-list'),
+    '/': () => requestFocusSearch(),
+    '?': () => window.dispatchEvent(new CustomEvent(SHOW_SHORTCUT_HELP_EVENT)),
+  })
+  return null
+}
+
 function App() {
   return (
     <ErrorBoundary>
       <QueryClientProvider client={queryClient}>
         <BrowserRouter>
+          <GlobalShortcuts />
           <div className="min-h-screen bg-surface-50">
             <Navigation />
             <main className="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-8" id="main-content">
@@ -76,6 +93,7 @@ function App() {
                 <Route path="*" element={<NotFound />} />
               </Routes>
             </main>
+            <ShortcutHelpBanner />
           </div>
         </BrowserRouter>
       </QueryClientProvider>