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>

.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?