feat(ui): Sprint 9 — F1 onboarding tour (4-step welcome)

Hand-rolled 4-step tour (no react-joyride) anchors to existing
[data-tour="<id>"] attributes. localStorage key
mealplanner:onboarding-complete is the source of truth; ?reset-tour=1
clears the key and re-shows.

Steps: Dashboard / Pantry / Recipes / Shopping List. Keyboard: 1-4 jump,
←/→ step, Esc dismiss. Off-route fallback renders a centered card with
an 'Open <page>' CTA. A11y: role=dialog, aria-modal=true, focus captured
on open and restored on close.

5 lines of code across 4 pages; 1 new component (~420 lines). No new
dependencies. No backend changes. No migration. Frontend-only deploy.

Tracking: Review/sprint9-verification.md (8-step browser smoke + a11y
check + reset-link test).

frontend/src/pages/Dashboard.tsx

@@ -599,7 +599,7 @@ export default function Dashboard() {
       </div>
 
       {/* Weekly Grid — all 7 days visible */}
-      <Card>
+      <Card data-tour="dashboard">
         <CardHeader>
           <h2 className="text-lg font-semibold text-surface-900">Weekly Overview</h2>
         </CardHeader>