feat: editor layout redesign v2 + 3D box select (#207)
Major layout redesign introducing v2 editor layout:
**New Features**
- V2 two-column layout (resizable sidebar + viewer panel with toolbar slots)
- 3D box select tool with marquee selection (561 lines pure Three.js geometry)
- View mode system (3D/2D/Split) replacing old floorplan toggle
- Floating level selector on viewer panel
- Viewer toolbar: level mode, wall mode, units, theme, camera, walkthrough, preview
- Horizontal tab bar for sidebar panels
**Layout Changes**
- Fixed 5-button control bar: Select, Box Select, Site Edit, Build, Delete
- Simplified view toggles (scans/guides only)
- Always-mounted viewers (display:none) to preserve WebGL context
- First-person walkthrough forces 3D view, restores on exit
- Site edit as permanent control (phase='site' is sole signal)
**Technical**
- layoutVersion prop: 'v1' (default) or 'v2'
- New exports: FloatingLevelSelector, SettingsPanel, SidebarTab, ViewMode, ViewerToolbarLeft/Right
- isCollapsed/setIsCollapsed added to sidebar store
- isFirstPersonMode restored for walkthrough support
19 files changed, +3230/-1898
Built and verified: bun run build passes (all tasks, 0 errors)
