Detect device add/remove at runtime (USB hotplug + Bluetooth connect/disconnect)

[ericfitz]

Problem

The app enumerates the foot switch and configures the listener once at launch
(AppDelegate.applicationDidFinishLaunching). It does not react to devices being
added or removed while running:

• USB: plugging/unplugging the pedal is not noticed.
• Bluetooth: connecting/disconnecting the FS17Pro is not noticed.

So the Settings device row (detected / config-verify) and the menu state reflect
only what was present at launch, and a user who switches the FS17Pro between USB and
Bluetooth modes after launch sees stale status until they relaunch.

(The runtime key listener itself is a global event tap and keeps working
regardless — this is specifically about device detection/status and any
transport-dependent programming UI being stale.)

Desired behavior

React to device add/remove live:

• USB: register IOKit HID matching/removal callbacks
  (IOHIDManagerRegisterDeviceMatchingCallback /
  IOHIDManagerRegisterDeviceRemovalCallback) instead of a one-shot
  IOHIDManagerCopyDevices, and refresh detection/status when they fire.
• Bluetooth: observe CoreBluetooth connect/disconnect
  (centralManager(_:didConnect:) / didDisconnectPeripheral) or re-query
  retrieveConnectedPeripherals on a trigger, and refresh.
• Refresh the Settings device row and any transport-dependent state when the
  connected transport changes (e.g. user switches the FS17Pro from USB to BLE).

Notes / relationship to current work

• Detection currently lives in FootswitchHIDController (USB IOKit) and
  BLEPedalProgrammer (CoreBluetooth, on-demand).
• The FS17Pro work adds transport-keyed trigger config and transport-aware
  programming (program writes the key for the currently-connected transport).
  Live transport detection would make that programming target update automatically
  when the user switches modes, instead of depending on launch-time state /
  manual Settings refresh.

Deferred from the FS17Pro Bluetooth support work.