feat: backport monorepo PRs #297, #302, #306 — floorplan thumbnails + mobile editor UI

Port floorplan item thumbnails, full mobile editor UI, and
mobile polish from the private monorepo into the public editor.

Monorepo PR #297 (feat/item-admin-advanced):
- Add optional floorPlanUrl to item asset schema (core)
- Render 2D floor-plan images inside item footprints on floorplan
- Improve slider drag with modifier key re-anchoring

Monorepo PR #302 (feat/mobile-ui):
- Mobile editor layout with draggable bottom sheet
- Mobile tab bar, selection bar, and panel sheet
- Mobile-aware panel manager and panel wrapper
- useIsMobile rewrite (useSyncExternalStore, SSR-safe)
- Camera actions hideOrbit prop
- GridSnapControl and SecondaryToggles exports
- Action menu hides on mobile contextual tabs
- SidebarTab extended with mobileDefaultSnap/mobileIcon

Monorepo PR #306 (feat/mobile-ui-polish):
- Touch gesture mapping for camera controls (one/two/three finger)
- Snap ratio constants for bottom sheet
- Thumbnail generator WebGL2 fallback (bottom-up row flip)
- ErrorBoundary scope logging, viewer scene wrap
- GPUDeviceWatcher enhanced logging and uncaptured error handler
- WebGPURenderer init error handling and diagnostics
- Post-processing log improvements
- MergedOutlineNode WebGL2 FBO corruption fix

Excluded: apps/community/*, apps/editor/*, packages/community-*,
.cursor/*, .env.example (monorepo-only files)

Co-authored-by: Pascal <open@pascal.app>

packages/core/src/schema/nodes/item.ts

@@ -80,6 +80,9 @@ const assetSchema = z.object({
   category: z.string(),
   name: z.string(),
   thumbnail: z.string(),
+  // Optional top-down 2D image shown inside the item's footprint on the
+  // floor plan. When present, replaces the default diagonal-cross marker.
+  floorPlanUrl: z.string().optional(),
   src: AssetUrl,
   dimensions: z.tuple([z.number(), z.number(), z.number()]).default([1, 1, 1]), // [w, h, d]
   attachTo: z.enum(['wall', 'wall-side', 'ceiling']).optional(),