feat: built-in volume management

- Volume slider on every node (green bar, draggable)
- Mute toggle button (M/m) on every node
- Backend: read volume/mute from PipeWire node props
- Backend: POST /api/volume {node_id, volume} to set volume
- Backend: POST /api/mute {node_id, mute} to toggle mute
- Graph JSON includes volume and mute fields per node
- Slider supports drag-to-adjust with mouse

src/graph_engine.h

@@ -44,6 +44,10 @@ public:
     bool connectPorts(uint32_t output_port_id, uint32_t input_port_id);
     bool disconnectPorts(uint32_t output_port_id, uint32_t input_port_id);
 
+    // Volume control
+    bool setNodeVolume(uint32_t node_id, float volume);
+    bool setNodeMute(uint32_t node_id, bool mute);
+
     // PipeWire internal data (exposed for C callbacks)
     struct PwData {
         pw_thread_loop *loop;