Improve 3D renderer: shadow camera, caching, error handling, labels

Renderer improvements:
- Configure shadow camera frustum to cover full house (was default -5..5)
- Add geometry and material caching to reduce GPU allocations
- Add proper disposal of Three.js objects on floor switch (fix memory leak)
- Add error handling for fetch failures with custom event dispatch
- Add room labels as sprites floating in each room
- Support wall-mounted furniture Y positioning via position.y
- Use cached highlight materials instead of mutating shared materials

src/index.html

@@ -100,6 +100,9 @@
       await renderer.loadDesign('../designs/sample-house-design.json');
       buildFloorButtons();
       buildRoomList();
+    }).catch(err => {
+      document.getElementById('house-name').textContent = 'Error loading data';
+      document.getElementById('info').textContent = err.message;
     });
 
     function buildFloorButtons() {