Add Item Catalog API

[great-majority]

This PR introduces a new item catalog system that allows external clients to browse and retrieve information about all available items in the Studio.

What's Added

• New item command type with three subcommands:
  • list-groups - Get all item groups with category counts
  • list-group - Get categories within a specific group
  • list-category - Get all items within a category with detailed properties
• ItemCommandHandler - Handles item catalog requests and returns structured data about available items, groups, and categories
• Comprehensive documentation - Updated both English and Japanese README files with examples and usage instructions

Why This is Useful

• Enables external tools to programmatically discover what items are available
• Provides detailed item properties (animation support, color slots, pattern support, etc.)
• Supports building dynamic UI for item selection in external applications
• Chunked data retrieval avoids WebSocket size limitations

Usage Example

// Get all groups
{"type": "item", "command": "list-groups"}

// Get items in a category
{"type": "item", "command": "list-category", "groupId": 0, "categoryId": 1}

The implementation integrates seamlessly with the existing WebSocket command architecture and maintains backward compatibility.