fix(sync): stop a local sync reporting itself as a background one

Reported from hand testing: a manual sync ended with "Sync finished
elsewhere" stamped over its own result.

Ownership of a lock period was being decided when the lock was RELEASED,
by asking MailSync::isRunning(). That question cannot be answered then:
the process exits, so isRunning() goes false, and only afterwards does
the next poll observe the lock gone. The guard therefore suppressed the
message while the sync ran and let it through at the end, up to two
seconds after onSyncFinished() had already said what happened.

Ownership is now latched when the lock APPEARS, which is the moment
isRunning() can still answer, and the matching release is swallowed.
onSyncFinished() hands the latch back when it sees exit 75, because a
skip means the lock was never ours: if a manual run and the cron run
start inside one poll interval, the lock would otherwise be latched as
local and that other run's completion swallowed with it.

Also renames the messages to "Background sync running/completed" per the
user: "finished elsewhere" reads as though the application does not know
what is syncing the Maildir, when in fact it is the same script.

The tests added here cover the external path and the Unknown state. They
do NOT reproduce the reported bug, and were checked against a reverted
fix to confirm that: staging it needs isRunning() true at the Running
transition and false at the Idle one, which cannot be arranged in
test_mainwindow without a configured sync command and a live child
process. That was tried and abandoned, it left a process running for the
length of the suite and popped a dialog. The ordering and the fix were
instead verified against a standalone model of both code paths.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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