feat(FN-XXXX): add fnBinaryCheckEnabled global setting

Defaults to true so existing behaviour (dashboard probes PATH for `fn` /
`fusion` and surfaces install / version-mismatch states) is unchanged.
When the user toggles it off in Settings → General → CLI Binary:

- `GET /system/fn-binary/status` short-circuits before `detectFnBinary()`
  and returns `state: "skipped"`. No `<bin> --version` subprocess is
  spawned at all, so an outdated globally-installed CLI cannot run as a
  side effect of opening the dashboard.
- The install banner is hidden when `state` is `"skipped"`.
- `POST /system/fn-binary/install` rejects with HTTP 409 since install
  is the user action that the status check informs.
- The Settings UI gains a checkbox under the existing CLI binary panel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/settings-schema.ts

@@ -37,6 +37,7 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   favoriteModels: undefined,
   openrouterModelSync: true,
   updateCheckEnabled: true,
+  fnBinaryCheckEnabled: true,
   updateCheckFrequency: "daily",
   showGitHubStarButton: true,
   modelOnboardingComplete: undefined,