flash-mac: enable udpMidiBroadcast by default in config.json
The UDP MIDI relay to the notepat-remote amxd plugin is gated on
/mnt/config.json.udpMidiBroadcast. Without it, notepat never calls
system.udp.sendMidi() — which means the newly-added "UDP MIDI" status
badge would permanently read "OFF" after any fresh flash and the amxd
would never receive notes, even with a healthy session-server WS
fanout.
Flash-mac now writes `udpMidiBroadcast: true` into both partitions'
config.json alongside the handle/sub/email identity block. Users who
want to opt out (privacy — every keystroke is broadcast via session
server fanout) can still edit the USB's config.json after flash and
flip it back to false.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>