fix: hide shader scenes on non-wgpu renderer tiers

The immersive shader scenes (Plasma/Tunnel/Aurora/Spectral Ribbon/Line
Bed) render through the wgpu underlay, which does not exist on the
femtovg-GL and software tiers - picking one there showed a black view.
Gate the five picker rows and the g-cycle behind a new
ImmersiveState.shader-scenes-available flag seeded from the renderer
selection; FOCUS views and splits (Slint-drawn) stay available.

cc

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

@@ -628,7 +628,10 @@ component ImmersiveViewMenuPopup inherits PopupWindow {
                     root.picked();
                 }
             }
-            ImmersiveMenuRow {
+            // Shader scenes need the wgpu underlay — on the femtovg-GL /
+            // software tiers they render black, so the whole group is hidden
+            // (ImmersiveState.shader-scenes-available is seeded from Rust).
+            if ImmersiveState.shader-scenes-available: ImmersiveMenuRow {
                 icon: @image-url("../assets/icons/chart-no-axes-column.svg");
                 label: @tr("Plasma");
                 active: root.is-focus && root.sh == 1;
@@ -637,7 +640,7 @@ component ImmersiveViewMenuPopup inherits PopupWindow {
                     root.picked();
                 }
             }
-            ImmersiveMenuRow {
+            if ImmersiveState.shader-scenes-available: ImmersiveMenuRow {
                 icon: @image-url("../assets/icons/disc-3.svg");
                 label: @tr("Tunnel");
                 active: root.is-focus && root.sh == 2;
@@ -646,7 +649,7 @@ component ImmersiveViewMenuPopup inherits PopupWindow {
                     root.picked();
                 }
             }
-            ImmersiveMenuRow {
+            if ImmersiveState.shader-scenes-available: ImmersiveMenuRow {
                 icon: @image-url("../assets/icons/audio-lines.svg");
                 label: @tr("Aurora");
                 active: root.is-focus && root.sh == 3;
@@ -655,7 +658,7 @@ component ImmersiveViewMenuPopup inherits PopupWindow {
                     root.picked();
                 }
             }
-            ImmersiveMenuRow {
+            if ImmersiveState.shader-scenes-available: ImmersiveMenuRow {
                 icon: @image-url("../assets/icons/chart-no-axes-column.svg");
                 label: @tr("Spectral Ribbon");
                 active: root.is-focus && root.sh == 4;
@@ -664,7 +667,7 @@ component ImmersiveViewMenuPopup inherits PopupWindow {
                     root.picked();
                 }
             }
-            ImmersiveMenuRow {
+            if ImmersiveState.shader-scenes-available: ImmersiveMenuRow {
                 icon: @image-url("../assets/icons/radio.svg");
                 label: @tr("Line Bed");
                 active: root.is-focus && root.sh == 5;
@@ -1191,9 +1194,12 @@ export component ImmersiveView inherits Rectangle {
             // Aurora -> Spectral Ribbon -> Line Bed -> Off). Liquid Spectrum (mode
             // 6) is a PARKED POC — to bring it back, change the Math.mod to 7 and
             // uncomment its picker row above. The header picker is the discoverable control.
+            // No-op on the GL/software tiers (no wgpu underlay = black scenes).
             if (event.text == "g" || event.text == "G") {
-                ImmersiveState.shader-mode = Math.mod(ImmersiveState.shader-mode + 1, 6);
-                root.wake();
+                if (ImmersiveState.shader-scenes-available) {
+                    ImmersiveState.shader-mode = Math.mod(ImmersiveState.shader-mode + 1, 6);
+                    root.wake();
+                }
                 return accept;
             }
             return reject;

crates/qbz-ui/ui/state.slint

@@ -1200,6 +1200,9 @@ export global LocalLibraryActions {
     // Albums bulk bar action: select-all | clear | queue | play-next |
     // add-to-playlist | add-to-mixtape.
     callback albums-bulk-action(string /* action */);
+    // Windowed grid: the visible row band changed (item indices into
+    // albums-visible, prefetch margin included) — dispatch artwork for it.
+    callback albums-window-changed(int /* first */, int /* last */);
 
     // --- Tracks tab ---
     callback tracks-search(string /* query */);
@@ -3797,6 +3800,10 @@ export global ImmersiveState {
     // 0->1->2->3->0 with the 'g' key.
     in property <image> shader-texture;
     in-out property <int> shader-mode: 0;
+    // False on the femtovg-GL / software renderer tiers (no wgpu underlay →
+    // shader scenes would render black). Seeded from Rust at startup; gates
+    // the picker rows and the `g` cycle in ImmersiveView.
+    in-out property <bool> shader-scenes-available: true;
 
     // --- Immersive search cortinilla (parallel to SearchState.cortinilla-*) ---
     // Albums/Artists/Playlists only; selecting ACTS ON PLAYBACK (per the Settings