Add InteractionManager with mode system, selection outline, keyboard shortcuts

- Mode system: view | select | move | rotate | place
- Click furniture to select with cyan wireframe outline
- Keyboard: R/Shift+R rotate, Delete remove, Escape deselect,
  Ctrl+Z undo, Ctrl+Shift+Z/Ctrl+Y redo
- Listens to DesignState changes to sync scene on undo/redo
- Wired into index.html with DesignState integration
- Added furnitureIndex to mesh userData for state lookups

src/renderer.js

@@ -575,6 +575,7 @@ export class HouseRenderer {
           isFurniture: true,
           catalogId: placement.catalogId,
           roomId: roomDesign.roomId,
+          furnitureIndex: i,
           itemName: catalogItem.name,
           wallMounted: !!placement.wallMounted
         };