wire up demo gallery drawer, rename SaveTemplateOverlay to ExportOverlay

- Add Demos button to header and DemoGallery drawer with category filter
- Send demo prompt to CopilotChat via appendMessage on "Try it" click
- Rename SaveTemplateOverlay → ExportOverlay, drop unused description prop
- Remove dead iframe preview code from DemoCard, remove unused html field
- Fix trailing blank line in agent system prompt
- Update chat suggestion from BFS/DFS to 3D Plane Controls
- Add clipboard write error handling in ExportOverlay