fix(desktop): keep the rustcode on PATH at the app's version #3411

Problem

The rustcode CLI that a shell runs is an older copy than the app's own engine, on both test machines. Version-skew checks between machines then compare the wrong binary, and rustcode server install from a shell installs the stale one.

Evidence (2026-11-10)

Windows 11 (10.0.26100, AMD64), app 1.28.0 installed per-user:

> rustcode --version                                  # C:\Users\<user>\.rustcode\bin\rustcode.exe (install script, self-updated 11/9)
rustcode 1.25.0
> & "$env:LOCALAPPDATA\RustCode\rustcode.exe" --version
rustcode 1.28.0                                       # uninstall entry DisplayVersion 1.28.0

macOS (Mac Studio), three copies:

/opt/homebrew/bin/rustcode            0.96.0   # brew formula lhagfoss/tap/rustcode, first on PATH
/Applications/RustCode.app/.../rustcode   1.28.0
~/.local/bin/rustcode                 1.28.0   # what the launchd server runs (`rustcode server status` → Binary)

Acceptance criteria

• rustcode server status (and the app's machine row) reports when the rustcode on PATH differs from the app's engine version, with the exact paths.
• The app offers one action that points PATH's rustcode at its bundled engine (or updates it), on macOS, Linux and Windows.
• The desktop app installer/updater does not leave an older ~/.rustcode/bin/rustcode.exe ahead of it on PATH silently.