chore: remove Unity AI Toolkit packages to fix enum warnings

- Remove com.unity.ai.assistant, com.unity.ai.generators, com.unity.ai.inference
- Update packages-lock.json
- Additional Phase 19 visual polish updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

unity-client/Packages/manifest.json

@@ -3,9 +3,6 @@
     "com.coplaydev.unity-mcp": "https://github.com/CoplayDev/unity-mcp.git?path=/MCPForUnity",
     "com.endel.nativewebsocket": "https://github.com/endel/NativeWebSocket.git#upm",
     "com.unity.2d.enhancers": "1.0.0",
-    "com.unity.ai.assistant": "1.0.0-pre.12",
-    "com.unity.ai.generators": "1.0.0-pre.20",
-    "com.unity.ai.inference": "2.4.1",
     "com.unity.feature.2d": "2.0.2",
     "com.unity.inputsystem": "1.17.0",
     "com.unity.multiplayer.center": "1.0.1",