Add ViewerToolbarLeft and ViewerToolbarRight components:
- 3D/2D/Split view mode segmented control
- Collapse sidebar button
- Level mode toggle (stacked/exploded/solo)
- Wall mode toggle (cutaway/full height/low)
- Measurement unit toggle (metric/imperial)
- Day/night theme toggle
- Perspective/orthographic camera toggle
- Walkthrough (first-person) mode
- Preview mode button

Wire up toolbar in the v2 layout app page.

- Add isCollapsed/setIsCollapsed to useSidebarStore (was missing from
  editor repo, present in upstream) — fixes CollapseSidebarButton crash
- Add collapse threshold logic to setWidth
- Restore isFirstPersonMode/setFirstPersonMode to editor store — was
  removed when syncing use-editor.tsx but still needed by
  custom-camera-controls and viewer toolbar walkthrough button
- Forces perspective camera + full-height walls when entering
  first-person mode

- Import and render FirstPersonControls inside the Viewer scene
- Add FirstPersonOverlay with exit button when in first-person mode
- Switch Viewer to selectionManager='default' during first-person mode
- Conditionally hide editor tools (selection, box select, grid, tool
  manager, site edge labels) during walkthrough
- Show ViewerZoneSystem + InteractiveSystem during first-person mode
- Hide camera controls hint during walkthrough
- V2 layout enters full-screen walkthrough view (same as preview mode
  but with FirstPersonControls instead of orbit)

When entering first-person mode from split or 2D view:
- Save current viewMode before switching
- Force viewMode to '3d' and close floorplan
- On exit, restore the previous viewMode (split/2d/3d)

Instead of swapping between EditorLayoutV2 and a full-screen walkthrough
view (which unmounts/remounts the Three.js canvas, causing material
traversal errors), keep the v2 layout always mounted and overlay the
FirstPersonOverlay on top via fixed positioning.

The viewMode: '3d' state change already hides the 2D pane, and the
conditional rendering in viewerSceneContent disables editor tools. The
overlay provides the crosshair, controls hint, and exit button.

The FirstPersonOverlay wrapper div had pointer-events-auto on the full
area, blocking clicks from reaching the Three.js canvas and preventing
pointer lock activation. Now only the exit button and crosshair/hint
elements have their own pointer-events settings — the rest passes
through to the canvas so clicking activates mouse look.