feat(FN-2663): add cached update-check setting, APIs, and banner

- Add global updateCheckEnabled setting to core schema/types and wire dashboard command to cache update checks in the CLI
- Implement dashboard server update-check cache module plus REST routes for status and refresh behavior
- Add dashboard client hook, legacy API helpers, and UpdateAvailableBanner UI to show cached CLI update notices
- Cover update-check server routes, hook behavior, banner rendering, and route registration with focused tests
- Document update-check configuration and API behavior in architecture and settings reference docs

packages/core/src/settings-schema.ts

@@ -29,6 +29,7 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   favoriteProviders: undefined,
   favoriteModels: undefined,
   openrouterModelSync: true,
+  updateCheckEnabled: true,
   modelOnboardingComplete: undefined,
   useClaudeCli: undefined,
   // Global baseline lanes for per-role model selection