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/dashboard/app/api/legacy.ts

@@ -410,6 +410,25 @@ export function updateSettings(settings: Partial<Settings>, projectId?: string):
   });
 }
 
+export interface UpdateCheckResponse {
+  currentVersion?: string;
+  latestVersion?: string | null;
+  updateAvailable: boolean;
+  lastChecked?: number;
+  disabled?: boolean;
+  error?: string;
+}
+
+export function checkForUpdate(projectId?: string): Promise<UpdateCheckResponse> {
+  return api<UpdateCheckResponse>(withProjectId("/update-check", projectId));
+}
+
+export function refreshUpdateCheck(projectId?: string): Promise<UpdateCheckResponse> {
+  return api<UpdateCheckResponse>(withProjectId("/update-check/refresh", projectId), {
+    method: "POST",
+  });
+}
+
 export interface RemoteSettings {
   remoteActiveProvider: "tailscale" | "cloudflare" | null;
   remoteTailscaleEnabled: boolean;