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

icons.py

@@ -279,6 +279,40 @@ def icon_solo(color: str = "#ffaa00") -> QIcon:
     return _make_icon(paint, color)
 
 
+def icon_mic(color: str = "#00ff41") -> QIcon:
+    def paint(p, r, c):
+        p.setPen(Qt.NoPen)
+        p.setBrush(c)
+        # Mic body - pill/rectangle shape
+        bw = r.width() * 0.35
+        bh = r.height() * 0.45
+        bx = r.center().x() - bw / 2
+        by = r.top() + r.height() * 0.05
+        p.drawRoundedRect(QRectF(bx, by, bw, bh), bw * 0.3, bw * 0.3)
+        # Mic arc at top
+        path = QPainterPath()
+        arc_w = bw * 0.6
+        arc_h = r.height() * 0.08
+        path.moveTo(r.center().x() - arc_w / 2, by)
+        path.quadTo(r.center().x(), by - arc_h, r.center().x() + arc_w / 2, by)
+        p.drawPath(path)
+        # Stand lines down from body
+        pen = QPen(c, r.width() * 0.06)
+        pen.setCapStyle(Qt.RoundCap)
+        p.setPen(pen)
+        p.setBrush(Qt.NoBrush)
+        stand_top = by + bh
+        p.drawLine(QPointF(r.center().x(), stand_top),
+                   QPointF(r.center().x(), stand_top + r.height() * 0.15))
+        # Base
+        base_w = r.width() * 0.45
+        base_h = r.height() * 0.1
+        p.drawRoundedRect(QRectF(r.center().x() - base_w / 2,
+                                 stand_top + r.height() * 0.12,
+                                 base_w, base_h), base_w * 0.15, base_w * 0.15)
+    return _make_icon(paint, color)
+
+
 def icon_cog(color: str = "#aaaaaa") -> QIcon:
     def paint(p, r, c):
         pen = QPen(c, r.width() * 0.1)