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.cpp

@@ -3,6 +3,10 @@
 #include <pipewire/pipewire.h>
 #include <spa/utils/result.h>
 #include <spa/utils/list.h>
+#include <spa/param/props.h>
+#include <spa/pod/pod.h>
+#include <spa/pod/builder.h>
+#include <spa/pod/iter.h>
 
 #include <cstring>
 #include <algorithm>
@@ -65,16 +69,30 @@ static void on_node_info(void *data, const struct pw_node_info *info) {
     info = pw_node_info_update((struct pw_node_info*)obj->info, info);
     obj->info = (void*)info;
 
+    auto *nobj = static_cast<GraphEngine::NodeObj*>(obj);
+
     if (info && (info->change_mask & PW_NODE_CHANGE_MASK_PROPS)) {
-        auto *nobj = static_cast<GraphEngine::NodeObj*>(obj);
         if (info->props) {
             const char *media_name = spa_dict_lookup(info->props, PW_KEY_MEDIA_NAME);
             if (media_name && strlen(media_name) > 0)
                 nobj->node.media_name = media_name;
+
+            // Read volume from props
+            const char *vol_str = spa_dict_lookup(info->props, "volume");
+            if (vol_str) {
+                nobj->node.volume = pw_properties_parse_float(vol_str);
+            }
+
+            // Read mute from props
+            const char *mute_str = spa_dict_lookup(info->props, "mute");
+            if (mute_str) {
+                nobj->node.mute = pw_properties_parse_bool(mute_str);
+            }
         }
-        nobj->node.changed = true;
-        nobj->node.ready = true;
     }
+
+    nobj->node.changed = true;
+    nobj->node.ready = true;
 }
 
 static const struct pw_node_events node_events = {
@@ -770,4 +788,65 @@ bool GraphEngine::disconnectPorts(uint32_t output_port_id, uint32_t input_port_i
     return found;
 }
 
+// ============================================================================
+// Volume control
+// ============================================================================
+
+bool GraphEngine::setNodeVolume(uint32_t node_id, float volume) {
+    if (!m_pw.loop) return false;
+
+    pw_thread_loop_lock(m_pw.loop);
+
+    NodeObj *nobj = findNode(node_id);
+    if (!nobj || !nobj->proxy) {
+        pw_thread_loop_unlock(m_pw.loop);
+        return false;
+    }
+
+    // Build Props param with volume
+    uint8_t buf[1024];
+    struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buf, sizeof(buf));
+
+    struct spa_pod *param = (struct spa_pod*)spa_pod_builder_add_object(&b,
+        SPA_TYPE_OBJECT_Props, SPA_PARAM_Props,
+        SPA_PROP_volume, SPA_POD_Float(volume));
+
+    pw_node_set_param((pw_node*)nobj->proxy,
+        SPA_PARAM_Props, 0, param);
+
+    nobj->node.volume = volume;
+    nobj->node.changed = true;
+
+    pw_thread_loop_unlock(m_pw.loop);
+    return true;
+}
+
+bool GraphEngine::setNodeMute(uint32_t node_id, bool mute) {
+    if (!m_pw.loop) return false;
+
+    pw_thread_loop_lock(m_pw.loop);
+
+    NodeObj *nobj = findNode(node_id);
+    if (!nobj || !nobj->proxy) {
+        pw_thread_loop_unlock(m_pw.loop);
+        return false;
+    }
+
+    uint8_t buf[1024];
+    struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buf, sizeof(buf));
+
+    struct spa_pod *param = (struct spa_pod*)spa_pod_builder_add_object(&b,
+        SPA_TYPE_OBJECT_Props, SPA_PARAM_Props,
+        SPA_PROP_mute, SPA_POD_Bool(mute));
+
+    pw_node_set_param((pw_node*)nobj->proxy,
+        SPA_PARAM_Props, 0, param);
+
+    nobj->node.mute = mute;
+    nobj->node.changed = true;
+
+    pw_thread_loop_unlock(m_pw.loop);
+    return true;
+}
+
 // end of graph_engine.cpp