Anchor ambient visualizer to viewport bottom; occlude via z-index not clip

Drop the --player-height bottom inset so the fixed visualizer fills the
viewport; the inset player bar no longer leaves a page-background gap. The
spacer now occludes via opaque page-surface + z-index. Visualizer no longer
reads --player-height, so spacer.ts coalescing is removed.

DeepDrftPublic.Client/Layout/MainLayout.razor.css

@@ -1,7 +1,13 @@
-/* Spacer to prevent content overlap */
+/* Spacer to prevent content overlap. position:relative + z-index:1 establishes a stacking context
+   that paints above the WaveformVisualizer backdrop (fixed, z-index:0), and the opaque page-surface
+   background makes the spacer read as solid page — occluding the visualizer where it sits in flow,
+   the same way the app bar covers the top. Theme-aware alias, so it inverts for free. */
 .player-spacer {
     width: 100%;
     flex-shrink: 0;
+    position: relative;
+    z-index: 1;
+    background: var(--deepdrft-page-surface);
 }
 
 .player-spacer.expanded {