test(web): improve admin-themes test quality for GET /admin/themes/:rkey
- Extract MANAGE_THEMES constant to reduce repetition
- Rename setupAuth → setupAuthenticatedSession to match admin.test.tsx pattern
- Remove unnecessary fetch mock from unauthenticated test
- Strengthen CSS overrides assertion to require co-location via regex
- Add colorScheme and second token assertions to happy-path test
- Restore strict "Access Denied" assertion on 403 test
- Add ATB-62 reference to CSS overrides test description