Extract shared HTML/CSS design into a common template and create
generalised auth success and web auth templates that work for both
node registration and SSH check authentication flows.
Updates #1850
Generalise the registration pipeline to a more general auth pipeline
supporting both node registrations and SSH check auth requests.
Rename RegistrationID to AuthID, unexport AuthRequest fields, and
introduce AuthVerdict to unify the auth finish API.
Add the urlParam generic helper for extracting typed URL parameters
from chi routes, used by the new auth request handler.
Updates #1850
Add test to validate HTML template output consistency across all
templates (OIDC callback, registration, Windows, Apple).
Verifies all templates produce valid HTML5 with:
- Proper DOCTYPE declaration
- HTML5 lang attribute
- UTF-8 charset
- Viewport meta tag
- Semantic HTML structure
Ensures template refactoring maintains standards compliance.
