refactor(desktop): extract deferred launch and release lifecycle, completing the updater split (#6139)

* refactor(desktop): extract deferred launch helpers into updater/deferred-launch

Mechanical move out of updater.ts: the embedded mac shell and Windows
PowerShell wait-for-pid installer-opening scripts, their writers and
spawn plumbing, and the payload after-quit launch move to
updater/deferred-launch.ts. updater.ts re-exports the three deferred
launch input/result types; orphaned imports pruned. Region-based cut
(the PS1 heredocs contain column-zero braces, so this slice moves as one
contiguous span).

* refactor(desktop): extract the release cleanup lifecycle into updater/release-lifecycle

Mechanical move out of updater.ts: the release cleanup.json descriptor
types and guards, deprecate/remove scanning with deferred-failure retry,
the mkdir-based lifecycle lock, and the .back directory sweep move to
updater/release-lifecycle.ts. The DesktopUpdater interface type stays in
updater.ts; orphaned imports pruned. This completes the split: updater.ts
is now the assembly layer plus the state machine and download/install
orchestration.

Author: PerishCode