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>

package-lock.json

@@ -19,7 +19,7 @@
       },
       "devDependencies": {
         "@eslint/js": "^9.39.4",
-        "@playwright/test": "^1.49.0",
+        "@playwright/test": "^1.58.2",
         "@tailwindcss/vite": "^4.0.0",
         "@testing-library/jest-dom": "^6.6.3",
         "@testing-library/react": "^16.3.2",
@@ -33,6 +33,7 @@
         "globals": "^17.4.0",
         "jsdom": "^25.0.1",
         "msw": "^2.12.14",
+        "playwright": "^1.58.2",
         "tailwindcss": "^4.0.0",
         "typescript": "^5.7.3",
         "typescript-eslint": "^8.56.1",