defer apple intelligence availability check from app initialization
to when the user actually tries to use the feature. accessing
SystemLanguageModel.default during early app startup causes SIGABRT
on macOS 26.x (tahoe) beta.
- remove availability check from default_post_process_providers()
- always include apple intelligence provider on macos arm64
- add checkAppleIntelligenceAvailable command for lazy checking
- show error message if apple intelligence unavailable when selected
- clear error on any dropdown selection for better UX
