feat(shell): metaball ambient + translucent bars/panels/controls, drop carousel fades

Smoke rounds 3-4 (owner feedback). Checkpoint during iteration.

Ambient (the big one): rewrote ambient.wgsl from translating gaussian blobs to a
METABALL field (r²/d² summed + smoothstep iso-surface) with 2-octave domain warp
— lobes fuse/stretch/split like an amoeba / lava lamp (the Cider motion that was
missing), faster clock, more contrast.

Surfaces made background-aware (translucent when the dynamic background is on):
- Search results view (root + toolbar), artist Network panel + its tab-row
  header (transparent to avoid double-translucent compounding), JUMP TO bar
  (JumpNavBar bar-bg passed in — can't import ShellState without a cycle).
- Discover toolbars + tab pill + BrowseHeaderTools; SearchResultsView toolbar.
- CircleAction (shared: playlist/album/artist/etc. action buttons) on-surface
  secondary fill goes glass; playlist Sort dropdown; QbzSelect closed control.
- New app-background-bar-alpha knob (0.3, env QBZ_BG_BAR_ALPHA): thin bars go
  MORE transparent than panels (surface-alpha 0.5).
- Content well: frosted tint (0.22) + 1px hairline border → distinct panel.
- Album/Artist header gradient auto-suppressed under the dynamic background
  (they clashed); header text stays light on the dynamic backdrop.
- Discover carousel edge-fade "shadows" hidden when active (Carousel /
  PlaylistCarousel / ArtistCarousel / PinnedCarousel).

Base: 76ea5a82. Prior checkpoint: 4e405338.

cc

crates/qbz-ui/ui/primitives/CircleAction.slint

@@ -7,7 +7,7 @@
 import { Theme } from "../foundation/semantic-colors.slint";
 import { QbzIcon } from "QbzIcon.slint";
 import { LoadingSpinner } from "LoadingSpinner.slint";
-import { TooltipState } from "../state.slint";
+import { TooltipState, ShellState, AppearanceState } from "../state.slint";
 
 export component CircleAction inherits Rectangle {
     in property <image> icon;
@@ -48,7 +48,11 @@ export component CircleAction inherits Rectangle {
     background: root.on-surface
         ? (root.primary
             ? (ta.has-hover ? Theme.accent-hover : Theme.accent)
-            : ((ta.has-hover || root.active) ? Theme.surface-hover : Theme.surface-elevated))
+            : ((ta.has-hover || root.active)
+                ? Theme.surface-hover
+                : (ShellState.app-background-active
+                    ? Theme.surface-elevated.with-alpha(AppearanceState.app-background-surface-alpha)
+                    : Theme.surface-elevated)))
         : (root.primary
             ? (ta.has-hover ? #ffffffd6 : #ffffff)
             : ((ta.has-hover || root.active) ? #ffffff3d : #ffffff24));