add MCP developer tool with multi-unit support

First developer tool for the XDJ-100SX project. Connects Claude Code
directly to the Pi over SSH — push skin files, take screenshots, restart
Mixxx, flash Pico firmware, and more without leaving the editor.

Available MCP tools:
- run_command, read_file, write_file, list_files
- push_skin, pull_skin, push_skin_file, pull_skin_file
- push_midi, pull_midi
- take_screenshot, navigate_panel
- restart_mixxx
- check (preflight: SSH, Mixxx, Pico, audio)
- pico_bootloader, pico_flash
- discover_units — scan network for all reachable XDJ Pi units
- select_unit — switch active connection mid-session (multi-unit support)

Also adds --about flag and TUI About modal with authors and credits,
and fixes scrolling/close behavior on Help and About modals.

By: Jeancarlo Cardoso de Faria Filho (jaianlab) <jaianlabworks@gmail.com>

README.md

@@ -41,3 +41,5 @@ Note it only works with the Raspberry Pi 3B+
 2025 Marc Monka
 
 2026 Markus Golec
+
+2026 Jeancarlo Cardoso de Faria Filho (jaianlab) — Raspberry Pi Pico port, MCP developer tool