feat(viewer): redesign viewer UI with dark toolbar and camera controls (#126)

Overhaul viewer overlay with a unified bottom toolbar, dark theme support,
theme toggle, camera orbit/top-view controls, and fix TypeScript array
access errors for level and wall mode cycling.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/viewer/src/store/use-viewer.ts

@@ -89,7 +89,7 @@ const useViewer = create<ViewerState>()(
       levelMode: "stacked",
       setLevelMode: (mode) => set({ levelMode: mode }),
 
-      wallMode: 'cutaway',
+      wallMode: 'up',
       setWallMode: (mode) => set({ wallMode: mode }),
 
       showScans: true,