Move favicon.png, style.css, and headscale.svg to hscontrol/assets/
and create a single assets.go file with all embed directives.
Update hscontrol/handlers.go and hscontrol/templates/general.go to
use the centralized assets package.
Replace html/template with type-safe elem-go templating for OIDC
callback page. Improves consistency with other templates and provides
compile-time safety. All UI elements and styling preserved.
