Implement proper context management in the KiCad MCP server:
Add dedicated context.py with typed KiCadAppContext class
Convert tools to access context instead of parameters
Implement caching for thumbnails
Add proper cleanup of resources on shutdown
Improve error handling with cancellation support
Implements a more reliable PCB thumbnail generation feature using two methods:
- Primary: pcbnew Python module for high-quality rendering
- Fallback: pcbnew_cli for environments without Python modules
Adds detailed progress reporting and comprehensive error handling.
Includes documentation in docs/thumbnail_guide.md.
