feat(web): #67 Phase 3 - Kenney sprite overlay + AI avatar coverage (#70)

* feat(web): #67 Phase 3 - Kenney sprite overlay + AI avatar coverage

**Issue #67 Phase 3**

- Kenney furniture sprites overlay: lamp, rug, plant, speaker
- AI avatars: xiaoai/xiaokai覆盖到旧key
- FurnitureSprites.ts loader

Build: ✅

* feat(web): #67 Phase 3 - Kenney sprite overlay + AI avatar coverage

**Issue #67 Phase 3**

- Kenney Isometric Furniture Kit full assets (CC0) in public/kenney/
- overlayFurnitureSprites(): lamp, rug, plant, speaker
- FurnitureSprites.ts loader integrated into PixiApp
- AI avatars: xiaoai/xiaokai覆盖旧key, xiaoqian/xiaohou新建base

Build: ✅

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Co-authored-by: Ubuntu <ubuntu@localhost.localdomain>

Author: WingedDragon