fix(qconnect): make SetVolume a supersedable pending action

Add `is_set_volume_action` to PendingQueueAction (set for CtrlSrvrSetVolume) and
complete it on the SrvrCtrlVolumeChanged echo, so rapid volume changes can
supersede the in-flight one instead of colliding on the single pending slot (the
"pending queue action already active" spam). The frontend adapter clears the
superseded pending before sending the next SetVolume (lands with the qbz-slint
controller changes).

cc

crates/qconnect-app/src/app.rs

@@ -173,6 +173,10 @@ where
                 command.command_type,
                 QueueCommandType::CtrlSrvrSetLoopMode
             ),
+            is_set_volume_action: matches!(
+                command.command_type,
+                QueueCommandType::CtrlSrvrSetVolume
+            ),
             is_set_active_renderer_action,
             expected_active_renderer_id: if is_set_active_renderer_action {
                 pending_active_renderer_id_from_payload(&command.payload)
@@ -1096,6 +1100,16 @@ fn session_management_event_completes_pending_action(
         return true;
     }
 
+    if pending.is_set_volume_action
+        && matches!(event_type, QueueEventType::SrvrCtrlVolumeChanged)
+    {
+        // Volume changes come back as session-management events without a
+        // stable action_uuid ack. Treat the first volume-changed echo as the
+        // completion signal so a rapid volume drag is not blocked behind the
+        // generic 10s pending timeout.
+        return true;
+    }
+
     if !pending.is_set_active_renderer_action {
         return false;
     }

crates/qconnect-core/src/pending.rs

@@ -13,6 +13,8 @@ pub struct PendingQueueAction {
     #[serde(default)]
     pub is_set_loop_mode_action: bool,
     #[serde(default)]
+    pub is_set_volume_action: bool,
+    #[serde(default)]
     pub is_set_active_renderer_action: bool,
     #[serde(default)]
     pub expected_active_renderer_id: Option<i32>,