Expose ui_layout and ai_enabled to mobile clients and add enable_ai endpoint (#983)

* Wire ui layout and AI flags into mobile auth

Include ui_layout and ai_enabled in mobile login/signup/SSO payloads,
add an authenticated endpoint to enable AI from Flutter, and gate
mobile navigation based on intro layout and AI consent flow.

* Linter

* Ensure write scope on enable_ai

* Make sure AI is available before enabling it

* Test improvements

* PR comment

* Fix review issues: test assertion bug, missing coverage, and Dart defaults (#985)

- Fix login test to use ai_enabled? (method) instead of ai_enabled (column)
  to match what mobile_user_payload actually serializes
- Add test for enable_ai when ai_available? returns false (403 path)
- Default aiEnabled to false when user is null in AuthProvider to avoid
  showing AI as available before authentication completes
- Remove extra blank lines in auth_provider.dart and auth_service.dart

https://claude.ai/code/session_01LEYYmtsDBoqizyihFtkye4

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

---------

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

app/controllers/sessions_controller.rb

@@ -260,7 +260,9 @@ class SessionsController < ApplicationController
           user_id: user.id,
           user_email: user.email,
           user_first_name: user.first_name,
-          user_last_name: user.last_name
+          user_last_name: user.last_name,
+          user_ui_layout: user.ui_layout,
+          user_ai_enabled: user.ai_enabled?
         ),
         expires_in: 5.minutes
       )