v0.1.0-alpha.20

What's new

The install flow on the Updates screen no longer changes anything on disk until the countdown elapses, so Esc actually cancels the update.

• Async download with progress bar. Click "Install now" no longer freezes the GUI on the HTTP fetch; a thread streams the binary into pending_update/ while the screen shows a progress bar.
• Esc cancels the whole update. Pressing Esc during the download or the 5-4-3-2-1 countdown discards the staged binary, restores the previous state, and shows "Update cancelled — nothing was changed." (Before, the swap happened before the countdown — Esc only skipped the restart.)
• Countdown phase is now a separate state. replace_binary runs only when the countdown actually ends; that's also when restart_self fires.

Internal: new InstallPhase enum on UpdateCheckController (Idle → Downloading → Countdown), spawn_install_download worker, mpsc receiver polled from the GUI tick. 21 lib tests green.

No CLI / config changes.