fix(immersive): persist volume on drag-end

The immersive VolumeBar only called set-volume (live) and never persist-volume, so a volume set in the immersive view was lost on restart/track-change. Adds a released(float) callback emitted on pointer-up, wired to NowPlayingState.persist-volume — parity with the NPB sliders. Pure .slint; persist-volume already exists in Rust.

cc

crates/qbz-ui/ui/immersive/ImmersiveView.slint

@@ -153,7 +153,8 @@ component TinyBar inherits Rectangle {
 component VolumeBar inherits Rectangle {
     in property <float> value: 0.0;        // 0..1
     in property <bool> locked: false;
-    callback changed(float);               // emits 0..1
+    callback changed(float);               // emits 0..1 — live (every drag tick)
+    callback released(float);              // emits 0..1 — drag-end only (persist)
 
     // The visual fill fraction: pinned to 100% when locked (matches Tauri).
     property <float> shown: root.locked ? 1.0 : Math.max(0.0, Math.min(1.0, root.value));
@@ -226,6 +227,13 @@ component VolumeBar inherits Rectangle {
                 root.changed(Math.max(0.0, Math.min(1.0, self.mouse-x / self.width)));
             }
         }
+        // Persist only the final value on drag-end (matches the NPB sliders'
+        // released -> persist-volume; live changes still fire via changed).
+        pointer-event(e) => {
+            if (!root.locked && e.kind == PointerEventKind.up) {
+                root.released(Math.max(0.0, Math.min(1.0, self.mouse-x / self.width)));
+            }
+        }
     }
 }
 
@@ -354,6 +362,11 @@ component ImmersivePlayerBar inherits Rectangle {
                 NowPlayingState.set-volume(v);
             }
         }
+        released(v) => {
+            if (!root.vol-locked) {
+                NowPlayingState.persist-volume(v);
+            }
+        }
     }
     Rectangle {
         x: root.width - 42px;