feat: add core PWA screens (auth, dashboard, purchases, products, alerts, settings)

Build all 8 primary screens for CAR-33 on top of the Phase 1 scaffold:
- Auth: login, register, forgot password with JWT flow and mock fallback
- Dashboard: triggered alerts banner, spending stats, price trend sparklines (Recharts), recent purchases
- Purchase History: store filter chips, paginated list with item previews
- Purchase Detail: receipt view with line items linking to product pages
- Products: search with instant filter, store price comparison badges
- Product Detail: 90-day price history chart (Recharts), store comparison table
- Store Comparison: ranked store cards with savings banner
- Price Alerts: triggered/watching sections, create form, progress bars, delete
- Coupons: expiration warnings, copy-to-clipboard coupon codes
- Account Linking: connect Meijer/Kroger/Target with status indicators
- Settings: profile, connected stores, notification toggles, theme switcher, sign out

Also adds:
- Mock data layer (src/lib/mock-data.ts) for demo/screenshot use
- StoreIcon component with store brand colors
- Code-split Recharts chunk (initial JS: 117KB, Recharts lazy: 498KB)
- All 48px+ touch targets, mobile-first Tailwind layout

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/lib/api.ts

@@ -0,0 +1,36 @@
+import { useAuthStore } from '../stores/auth.ts'
+
+const API_BASE = import.meta.env.VITE_API_URL ?? '/api/v1'
+
+async function apiFetch<T>(path: string, options?: RequestInit): Promise<T> {
+  const token = useAuthStore.getState().token
+
+  const res = await fetch(`${API_BASE}${path}`, {
+    ...options,
+    headers: {
+      'Content-Type': 'application/json',
+      ...(token ? { Authorization: `Bearer ${token}` } : {}),
+      ...options?.headers,
+    },
+  })
+
+  if (res.status === 401) {
+    useAuthStore.getState().logout()
+    throw new Error('Unauthorized')
+  }
+
+  if (!res.ok) {
+    throw new Error(`API error: ${res.status}`)
+  }
+
+  return res.json() as Promise<T>
+}
+
+export const api = {
+  get: <T>(path: string) => apiFetch<T>(path),
+  post: <T>(path: string, body: unknown) =>
+    apiFetch<T>(path, { method: 'POST', body: JSON.stringify(body) }),
+  put: <T>(path: string, body: unknown) =>
+    apiFetch<T>(path, { method: 'PUT', body: JSON.stringify(body) }),
+  delete: <T>(path: string) => apiFetch<T>(path, { method: 'DELETE' }),
+}