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>