Update AppHeader to reflect authentication state. When not authenticated,
show Logga in and Registrera links. When authenticated, show the user's
email address and a Logga ut button. Uses v-if/v-else with template blocks
for clean conditional rendering without wrapper elements.
Changes:
- authStore: add email computed that extracts sub claim from JWT payload
- AppHeader: conditional nav with v-if/v-else (guest vs authenticated)
- AppHeader: add email display and logout button with styles
- App.spec.ts: add Pinia to test setup (required by AppHeader now)
- AppHeader.spec.ts: rewrite with tests for both auth states
- authStore.spec.ts: add tests for email extraction and clearing
- header-auth.spec.ts: 5 Playwright E2E tests for header auth state
