fix: solve #3507 — macOS Update button silently fails after repeated clicks

[github-actions]

Root cause

installUpdate() in apps/desktop/src/main/lib/auto-updater.ts had no
guard, so every click on the Install button fired another
autoUpdater.quitAndInstall(false, true).

On macOS, MacUpdater.quitAndInstall() re-registers a native-updater
update-downloaded listener every time it is called before Squirrel.Mac
has finished downloading. Each extra click stacks another listener —
and when Squirrel finally fires the event, every queued listener ends
up calling nativeUpdater.quitAndInstall() in parallel. Racing the
native updater like that can leave the downloaded binary un-swapped,
which matches the user report on #3507: the app closes after clicking
Update but relaunches on the old version.

The fix

• Add an isInstalling guard so repeated clicks short-circuit instead of
  re-entering autoUpdater.quitAndInstall().
• Only proceed when currentStatus === READY; otherwise log and no-op
  (nothing to install).
• Reset the guard on the autoUpdater error event so a failed attempt
  can be retried without restarting the app.

Tests

Reproduction test in apps/desktop/src/main/lib/auto-updater.test.ts:

• ignores install requests when no update is ready — clicking Install
  before the download finishes no longer calls quitAndInstall.
• does not invoke quitAndInstall multiple times when the install button is clicked repeatedly — three rapid clicks now result in exactly one
  quitAndInstall call (pre-fix: three calls, reproducing the race).

Verified that both tests fail on main without the fix and pass with
the fix applied.

Closes #3507

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Summary by cubic

Fixes #3507: Prevents the macOS Update button from failing after repeated clicks by guarding installs and only running when an update is ready. This stops electron-updater listener fan-out that caused relaunching on the old version.

Bug Fixes

• Add isInstalling guard to ignore duplicate clicks and avoid parallel quitAndInstall() calls.
• Require currentStatus === READY; otherwise log and no-op.
• Clear the guard on autoUpdater error to allow retry; add tests for duplicate clicks and not-ready installs.

^{Written for commit cd2f8b2. Summary will update on new commits.}