Auto-update "Restart failed" every time — cannot self-update, must reinstall manually (macOS arm64)

### Short summary

_No response_

### Affected version or release

~2.94.0 (macOS 15.7.7, Apple Silicon/arm64)

### Installation context

GitHub Copilot desktop app on macOS (personal machine)

### What happened?

The app shows a "Restart to update" prompt. After pressing Restart, it shows "Restart failed" and I must reinstall manually.

### Steps to reproduce

1. Update prompt appears
2. Click Restart
3. "Restart failed" toast appears

### Expected behavior

Restart applies the update and relaunches into the new version.

### Additional context

Reproduces every update. Likely updater cannot replace the app bundle.

Reproduce the update flow on macOS arm64 using the three reported steps, then investigate why Restart fails to apply the update and relaunch the app. The work is done when an update completes through the Restart prompt and the app opens in the new version without manual reinstallation.

Written by the indexing model from the issue text.