Fix left sidebar default width

[ComputelessComputer]

Keep the untouched left sidebar default tied to the rendered body width and cover startup resize behavior with a regression test.

---

Note

Low Risk
Localized desktop shell layout logic with guarded default-size tracking; no auth, data, or API changes.

Overview
Fixes the classic main left sidebar so its default/min/max panel percentages stay tied to the rendered body width (via getBoundingClientRect), not just window.innerWidth on first paint.

On mount and when the body resizes (resize + ResizeObserver), syncDefaultLeftSidebarPanelSize recomputes constraints from ~200px defaults and calls panel.resize only while the user is still at the untouched default. Manual resize or drag turns off that tracking; handlePanelLayout also ignores intermediate layouts until a pending programmatic resize settles (epsilon compare).

Tests mock body width and rAF to cover startup settle, window resize, ignoring stale onLayout values, and syncing after the sidebar panel appears when leaving onboarding.

^{Reviewed by Cursor Bugbot for commit 6c0e3ec. Bugbot is set up for automated code reviews on this repo. Configure here.}

[netlify]

✅ Deploy Preview for old-char canceled.

Name	Link
🔨 Latest commit	6c0e3ec
🔍 Latest deploy log	https://app.netlify.com/projects/old-char/deploys/6a450a57943e3000086c8c41

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit d4f5dde. Configure here.}