feat(visualizer): add stereo scopes

Add a theme-aware goniometer and pitch-locked oscilloscope to the large dock and immersive focus modes.

Gate scope DSP and scenegraph updates by actual panel visibility so hidden modes remain idle, including while shader scenes cover immersive panels.

cx

crates/qbz-qt/qml/shell/SidebarNowPlayingDock.qml

@@ -4,14 +4,13 @@
 // Mounted as a ROOT overlay by AppShell, pinned flush to the window
 // bottom-left (NOT inside the sidebar — that floated it above the bar).
 // Layout, bottom-up: a square album cover (= the fed width) at the very
-// bottom; ABOVE it a compact spectrum band the user can hide with the
-// top-right eye button on the cover.
+// bottom; ABOVE it either the compact spectrum band or a square scope the
+// user can hide with the top-right eye button on the cover.
 //
 // HEIGHT IS LOAD-BEARING. `QbzShell.largeDockHeight` (shell_bridge.rs) is the
 // single source of truth and the layout below is derived from the SAME
-// constants:
-//   ON  = 9 top pad + 42 band + 10 gap + 208 art + 4 bottom gap = 273
-//   OFF = 9 top pad                    + 208 art + 4 bottom gap = 221
+// constants and the persisted render mode: legacy modes are 42px high;
+// scope modes use the same square size as the artwork and grow upward.
 // Sidebar.qml reserves `largeDockHeight - npb height` so the playlist list
 // stops ABOVE the cover, and AppShell pins this overlay at
 // `height - largeDockHeight`. Change the arithmetic in one place and the
@@ -42,7 +41,8 @@ Item {
     property bool ambientOn: false
 
     readonly property bool bandOn: QbzShell.largeVisualizerOn
-    readonly property int bandHeight: 42
+    readonly property int bandHeight: QbzShell.largeSpectrumMode >= 3
+        ? Math.round(root.width) : 42
     readonly property int padTop: 9
     readonly property int bandGap: 10
     // The cover's top edge: below the band when it is shown.