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/pages/Pantry.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useRef, useState } from 'react'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { Plus, Search, Trash2, Package, AlertTriangle } from 'lucide-react'
 import { mealPlannerApi } from '../api'
@@ -10,6 +10,7 @@ import { Select } from '../components/ui/Select'
 import { Skeleton, SkeletonText } from '../components/ui/Skeleton'
 import { EmptyState } from '../components/ui/EmptyState'
 import { showToast, showApiError } from '../lib/toast'
+import { useFocusSearchOnShortcut } from '../hooks/useFocusSearch'
 
 const AISLE_OPTIONS = [
   { value: '', label: 'Select aisle…' },
@@ -21,6 +22,8 @@ export default function Pantry() {
   const [showAddForm, setShowAddForm] = useState(false)
   const [removeId, setRemoveId] = useState<string | null>(null)
   const [searchQuery, setSearchQuery] = useState('')
+  const searchInputRef = useRef<HTMLInputElement>(null)
+  useFocusSearchOnShortcut(searchInputRef)
 
   /* ingredient name typed by user */
   const [ingredientName, setIngredientName] = useState('')
@@ -272,6 +275,7 @@ export default function Pantry() {
         <div className="relative">
           <Search className="absolute left-3 top-1/2 -translate-y-1/2 w-4 h-4 text-surface-400" />
           <Input
+            ref={searchInputRef}
             value={searchQuery}
             onChange={(e) => setSearchQuery(e.target.value)}
             placeholder="Search pantry items..."