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)

apps/editor/app/page.tsx

@@ -1,11 +1,30 @@
 'use client'
 
-import { Editor } from '@pascal-app/editor'
+import {
+  Editor,
+  type SidebarTab,
+  ViewerToolbarLeft,
+  ViewerToolbarRight,
+} from '@pascal-app/editor'
+
+const SIDEBAR_TABS: (SidebarTab & { component: React.ComponentType })[] = [
+  {
+    id: 'site',
+    label: 'Scene',
+    component: () => null, // Built-in SitePanel handles this
+  },
+]
 
 export default function Home() {
   return (
     <div className="h-screen w-screen">
-      <Editor projectId="local-editor" />
+      <Editor
+        layoutVersion="v2"
+        projectId="local-editor"
+        sidebarTabs={SIDEBAR_TABS}
+        viewerToolbarLeft={<ViewerToolbarLeft />}
+        viewerToolbarRight={<ViewerToolbarRight />}
+      />
     </div>
   )
 }