Check for pending invitations before creating new Family during SSO log in/sign up (#1171)

* Check for pending invitations before creating new Family during SSO account creation

When a user signs in via Google SSO and doesn't have an account yet, the
system now checks for pending invitations before creating a new Family.
If an invitation exists, the user joins the invited family instead.

- OidcAccountsController: check Invitation.pending in link/create_user
- API AuthController: check pending invitations in sso_create_account
- SessionsController: pass has_pending_invitation to mobile SSO callback
- Web view: show "Accept Invitation" button when invitation exists
- Flutter: show "Accept Invitation" tab/button when invitation pending

https://claude.ai/code/session_019Tr6edJa496V1ErGmsbqFU

* Fix external assistant tests: clear Settings cache to prevent test pollution

The tests relied solely on with_env_overrides to clear configuration, but
rails-settings-cached may retain stale Setting values across tests when
the cache isn't explicitly invalidated. Ensure both ENV vars AND Setting
values are cleared with Setting.clear_cache before assertions.

https://claude.ai/code/session_019Tr6edJa496V1ErGmsbqFU

---------

Co-authored-by: Claude <noreply@anthropic.com>

test/controllers/settings/hostings_controller_test.rb

@@ -210,6 +210,9 @@ class Settings::HostingsControllerTest < ActionDispatch::IntegrationTest
         delete disconnect_external_assistant_settings_hosting_url
 
         assert_redirected_to settings_hosting_url
+        # Force cache refresh so configured? reads fresh DB state after
+        # the disconnect action cleared the settings within its own request.
+        Setting.clear_cache
         assert_not Assistant::External.configured?
         assert_equal "builtin", users(:family_admin).family.reload.assistant_type
       end