fix: auto-start registry write fails when running elevated as admin

app.getLoginItemSettings() always returns openAtLogin=false when the
process is elevated on Windows, causing a false permission error.
Replaced with direct reg.exe calls to HKCU\...\Run which work correctly
regardless of elevation level.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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