Remove solo buttons, hide mute on mics, fix master volume, add mic_on MIDI, resize fonts

- Removed solo buttons from all mixer channels
- Mute button hidden on mic channels (0,1) — mic_on toggle replaces it
- Fixed master volume fader not being applied in audio _process()
- Added mixer_mic_on MIDI signal and learnable action
- Added CLEAR MIDI button to settings popup
- Increased settings popup fonts 7pt→9pt, popup size 460→620px
- Reduced deck fonts (header 24→16, elapsed 32→22, etc.) and spacing
- Increased mixer fonts (names 8→9, headers 9→11, etc.)
- Added missing MIDI learn entries (mute ch3-8, pfl ch3-8, mic_on ch1-2)

midi_engine.py

@@ -94,6 +94,7 @@ class MidiEngine(QObject):
     mixer_mute           = Signal(int, bool)
     mixer_solo           = Signal(int, bool)
     mixer_pfl            = Signal(int, bool)
+    mixer_mic_on         = Signal(int, bool)
     master_control       = Signal(str, float)
     master_volume        = Signal(float)
     controller_connected = Signal(bool, str)
@@ -282,6 +283,9 @@ class MidiEngine(QObject):
         elif action == 'mixer_pfl':
             on = (value > 63) if not inv else (value < 64)
             self.mixer_pfl.emit(target_id, on)
+        elif action == 'mixer_mic_on':
+            on = (value > 63) if not inv else (value < 64)
+            self.mixer_mic_on.emit(target_id, on)
         elif action == 'master_control':
             knob = str(target_id)
             if is_rel and 1 <= value <= 63: