Phase 8: In-App AI Chat Panel

## Goal
Add an in-app AI assistant that can control the editor via natural language.

- QML panel connected to LLM
- Natural language commands: "make the selected object twice as large", "create a metallic material"
- LLM generates MCP tool calls executed internally
- Reuses existing LLMManager + MCPServer infrastructure

**Depends on:** Phase 7
**Part of:** UX Redesign Roadmap