Restore --cron warning visibility; show destination on blob upload

--cron used to behave like full quiet mode for everything that
wasn't an error: warnings were swallowed both in the structured
log channel (LevelError gate) and at the snapshot terminus (the
"Finished (with N warnings)" line went through ui.Complete, which
is silenced under SetQuiet). A backup that, say, hit Full Disk
Access permission errors on a handful of files and skipped them
via --skip-errors would exit 0 and emit nothing — the cron job
would never page anyone.

--cron now obeys "silent only on total success":

  * log.Initialize raises the cron/quiet log level from Error to
    Warn so log.Warn output still reaches stdout (and therefore
    cron's mail).
  * The post-backup terminus message switches to ui.Warning when
    WarningCount > 0. Warning is not silenced by SetQuiet, so cron
    delivers the summary line whenever the count is non-zero. The
    no-warnings path keeps ui.Complete, which IS silenced under
    cron — that's the success path.

Separately, blob upload UI now names the actual destination
instead of the generic "backup destination store" string. The
Begin/Info lines emit the storer's reported Location (s3://bucket,
file:///mnt/usb/backup, rclone://remote/path, etc.), so anyone
watching a backup can see exactly where each blob is landing.

internal/vaultik/snapshot.go

@@ -92,8 +92,13 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 		}
 	}
 
+	// Terminus must obey the --cron invariant: silent on total
+	// success only. UI.Complete is dropped in cron/quiet mode (that's
+	// the success path), but if any warnings fired during the run we
+	// emit the summary via UI.Warning so cron actually delivers
+	// something for the user to look at.
 	if v.UI.WarningCount() > 0 {
-		v.UI.Complete("Finished (with %d warnings).", v.UI.WarningCount())
+		v.UI.Warning("Finished with %d warning(s) — review the output above.", v.UI.WarningCount())
 	} else {
 		v.UI.Complete("Finished successfully.")
 	}