Fix Settings updates interrupted by plugin reloads

[gitcoder89431]

Settings' Update button could pull and deploy new files, then lose its updater when deployment reloaded the Settings plugin. The update stopped before the final shell restart, leaving stale UI and an interrupted lifecycle journal; users had to run omarchy restart shell manually.

Run the entire GUI update in a separate session with independent stdio, while retaining a relay for errors while Settings is alive. Save the latest output to ~/.local/state/ruixen/settings-update.log and reject concurrent GUI updates without truncating the active log. Continue using the existing updater and lifecycle lock. Document the behavior and lifecycle rule.

Validation:

• Added a regression test that kills the launching process group before completion and verifies the worker survives, plus concurrency and failure propagation checks.
• ./tests/run-all.sh passed.
• ./install.sh --dry-run and omarchy plugin validate ruixen.launcher passed.
• Installed locally, confirmed a full shell restart with a new PID, verified IPC response and About page loading, and checked startup logs for missing QML types or plugin fallback.