fix(desktop): update an older rustcode in ~/.rustcode/bin when the app updates #3487

Problem

#3411 is closed by #3486, which adds rustcode server link-path and a "Use this app's version in a terminal" button on the Machines page. One acceptance criterion is still open: an older rustcode in ~/.rustcode/bin, left by an earlier install, is not updated when the app updates. The app only reports the version mismatch and offers the button.

Expected

When the desktop app installs or updates, it also updates or re-links a rustcode in ~/.rustcode/bin that the app installed earlier. A Homebrew copy is still left to brew upgrade, and dev builds are refused, as link-path already does.

Acceptance criteria

• After an app update, rustcode --version in a new shell matches the app, when the shell finds ~/.rustcode/bin/rustcode.
• The Homebrew and dev-build exclusions from fix(desktop): keep the rustcode on PATH at the app's version #3486 still apply.
• A test covers the choice between update, skip for Homebrew, and skip for a dev build.