feat: add SlashCommand message type for Claude Code command blocks (#102)

Add support for parsing and displaying XML command blocks from Claude Code
JSONL files (e.g., /clear, /help, /model commands).

Changes:
- Add SlashCommand variant to MessageType enum
- Add SlashCommandData struct with command_name, message, args, stdout fields
- Add regex parsing in claude_code.rs for XML command blocks
- Add database migration 017 for slash_command message type
- Add yellow/amber styling in TUI for slash command messages
- Add SlashCommandMessage component in React GUI
- Add unit tests for slash command parsing
- Fix clippy large_enum_variant warning by boxing MessageGroup fields

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

Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: sanggggg