Eliminate code duplication in OIDC identity creation (#230)

* Eliminate duplication by using create_from_omniauth method

- Updated OidcIdentity.create_from_omniauth to set last_authenticated_at
- Refactored OidcAccountsController to use create_from_omniauth instead of direct create! calls
- Updated test to verify last_authenticated_at is set by create_from_omniauth

Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>

* Extract auth hash building into private helper method

- Added build_auth_hash helper method to eliminate OpenStruct creation duplication
- Both create_link and create_user actions now use the same helper

Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>

* Linter fix

* Fix button style on OIDC link step

* Fix dark mode styles

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jjmata <187772+jjmata@users.noreply.github.com>
Co-authored-by: Juan José Mata <juanjo.mata@gmail.com>

app/models/oidc_identity.rb

@@ -21,7 +21,8 @@ class OidcIdentity < ApplicationRecord
         name: auth.info&.name,
         first_name: auth.info&.first_name,
         last_name: auth.info&.last_name
-      }
+      },
+      last_authenticated_at: Time.current
     )
   end
 end