Add a one-tap audio mute on the VFO and Channel screens:
- SK2 short-tap toggles mute via the built-in audioAmpMute()/audioAmpIsMuted().
The tap is only consumed by the existing "set TX TG to incoming" behaviour
while a DMR QSO is on screen, so mute works everywhere else.
- Header: a bold "M" is shown just left of the battery while muted (the
firmware's built-in strike-through over the mode text also still shows).
- LED: flashes red for 0.5s every 2s while muted (main loop), overriding the
normal RX/TX LED.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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