refactor: move guides from left toolbar to settings

- Remove guides from LeftToolbar pane type buttons
- Remove 'guides' from PaneType union and PANE_LABELS
- Remove GuidesPane from pane exports and ThreePanelLayout rendering
- Add migration to reset persisted slots that had type 'guides'
- Guides remain accessible via Settings modal (GuidesViewer tab)

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

src/components/panes/index.ts

@@ -1,5 +1,4 @@
 export { default as NodePane } from './NodePane';
-export { default as GuidesPane } from './GuidesPane';
 export { default as DimensionsPane } from './DimensionsPane';
 export { default as MapPane } from './MapPane';
 export { default as ViewsPane } from './ViewsPane';