feat(e2e): add J1 and J8 journey tests

feat(e2e): add J1 and J8 journey tests

- J1: Registration and Login — register flow, validation errors,
  sign-in with existing account, nav between pages
- J8: Unauthenticated Access — /, /purchases, /products, /coupons
  all redirect to /login when no session
- Enable VITE_MOCK_AUTH in playwright webServer so registration
  tests work without a live Better-Auth instance
- Add playwright to devDependencies to ensure CI has the package

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

playwright.config.ts

@@ -9,7 +9,7 @@ export default defineConfig({
     },
   ],
   webServer: {
-    command: 'npm run dev',
+    command: 'VITE_MOCK_AUTH=true npm run dev',
     url: 'http://localhost:5173',
     reuseExistingServer: !process.env.CI,
   },