Add mic ON/OFF toggle, headphone and studio output busses

- Channels 0-1 are now dedicated Mic 1/Mic 2 with mic_on toggle
- Mic audio only routes to main when mic_on=True, never to headphone
- New headphone_out_L/R JACK ports (all channels except mics)
- New studio_out_L/R JACK ports (copies main, mutes when any mic is on)
- Mic ON/OFF toggle button in mixer UI (hidden on non-mic channels)
- Session save/load includes mic_on state

main.py

@@ -196,6 +196,7 @@ class RadioPanelWindow(QMainWindow):
                 'muted': ch.muted,
                 'solo': ch.solo,
                 'pfl': ch.pfl,
+                'mic_on': ch.mic_on,
             })
 
         os.makedirs(os.path.dirname(path), exist_ok=True)
@@ -226,6 +227,7 @@ class RadioPanelWindow(QMainWindow):
                 ch.muted = ch_state.get('muted', ch.muted)
                 ch.solo = ch_state.get('solo', ch.solo)
                 ch.pfl = ch_state.get('pfl', ch.pfl)
+                ch.mic_on = ch_state.get('mic_on', ch.mic_on)
 
         self.mixer.refresh_strips()