Critical: handleShutdownAfterFinish() captured shutdown mode in a
closure at scheduling time — changing mode during countdown was ignored,
causing unexpected system shutdown/restart/sleep.
Now reads shutdownMode at execution time, clears timer when mode
changes to 'nothing', clears orphaned timers before creating new ones,
and adds error handling on exec() calls.
Also: guard stats timer against double-start in upload-manager.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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