oidc: add confirmation page for node registration

Render an interstitial showing device hostname, OS, and machine-key
fingerprint before finalising OIDC registration. The user must POST
to /register/confirm/{auth_id} with a CSRF double-submit cookie.
Removes the TODO at oidc.go:201.

hscontrol/app.go

@@ -485,6 +485,7 @@ func (h *Headscale) createRouter(grpcMux *grpcRuntime.ServeMux) *chi.Mux {
 
 	if provider, ok := h.authProvider.(*AuthProviderOIDC); ok {
 		r.Get("/oidc/callback", provider.OIDCCallbackHandler)
+		r.Post("/register/confirm/{auth_id}", provider.RegisterConfirmHandler)
 	}
 
 	r.Get("/apple", h.AppleConfigMessage)