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)

audio_engine.py

@@ -532,6 +532,13 @@ class AudioEngine:
         master_peak = max(np.max(np.abs(master_audio[0])), np.max(np.abs(master_audio[1])))
         self.mixer['master'].vu_peak = master_peak
 
+        # Apply master volume fader (master bus only – headphone and studio
+        # maintain independent levels)
+        vol = self.mixer['master'].volume
+        if vol < 1.0:
+            master_buf_L[:] *= vol
+            master_buf_R[:] *= vol
+
         master = self.mixer['master']
         master._lufs_accum += frames
         if master._lufs_accum >= 512: