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  spawned at all, so an outdated globally-installed CLI cannot run as a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/dashboard/app/api/legacy.ts

@@ -1309,7 +1309,7 @@ export interface FnBinaryStatus {
     invocation: string;
   };
   expectedVersion: string;
-  state: "installed" | "missing" | "version-mismatch";
+  state: "installed" | "missing" | "version-mismatch" | "skipped";
   install: { npm: string; curl: string; package: string };
 }