[GUI] Create LED widget for MIDI activity indicators

[tensquaresoftware]

Create a LED widget class and add visual indicators in HeaderPanel to show incoming and outgoing MIDI message activity.

Tasks:

• Design and implement LED widget class (tss::Led)
• Add visual states: off, on, blinking
• Integrate LED instances in HeaderPanel for MIDI In
• Integrate LED instances in HeaderPanel for MIDI Out
• Wire LED updates to MIDI message events
• Test visual feedback with real MIDI traffic

Context:
Visual feedback helps users verify that MIDI communication is working correctly with the Matrix-1000.