Move autostart_*.go and tests from src/core to src/platform/autostart.
Update src/gui/app.go to call autostart.SetAutostart / autostart.AutostartStatus.
Add quoteDesktopExec to desktop_linux.go (was co-located in autostart_linux.go).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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