feat(now-playing): mount real waveform visualizer in NowPlaying card (mode C) + Fill container-sizing mode

Replace the 20 synthetic bars with a contained WaveformVisualizer driven by the live player, pointed at the current track; add a Fill mode (CSS-only, defaults off) sizing the canvas to its container; place the lava-lamp icon to popover on the card.

DeepDrftPublic.Client/Controls/WaveformVisualizer.razor.css

@@ -19,6 +19,19 @@
     overflow: hidden;
 }
 
+/* Fill / container-sizing mode (Phase 12 §6c, mode C). The contained hosts (the NowPlaying card)
+   set Fill="true": the canvas fills its nearest positioned ancestor instead of the viewport, and the
+   footer clip drops (there is no player bar to clear — the bounding box IS the clip). The host is
+   responsible for giving this layer a positioned, sized parent. `inset: 0` over `position: absolute`
+   makes the box exactly that parent's content box; `overflow: hidden` still clips the canvas to it.
+   The canvas backing-store sizing in WaveformVisualizer.ts is unchanged — it already measures the
+   canvas's own box, so this purely re-parents the box from the viewport to the container. */
+.mix-waveform-bg--fill {
+    position: absolute;
+    inset: 0;
+    z-index: 0;
+}
+
 /* The canvas fills the viewport. All ribbon shading (luminous depth, soft edges) is drawn inside the
    canvas by the WebGL2 fragment shader. NO CSS backdrop-filter: it was a confirmed per-frame perf killer
    on the Canvas predecessor and is exactly the cost the GPU move exists to eliminate (spec §2, §5.2);