feat(shell): per-GPU selector, ambient scene in immersive, glass carousels/viewall

Smoke round 5 (owner feedback). Checkpoint.

- Preferred GPU selector (Settings > Renderer, Linux/wgpu): lists each detected
  GPU by name ("Auto" + e.g. "NVIDIA … (discrete)" / "Intel … (integrated)"),
  persisted (ui_prefs.gpu_power by device name), applied at startup via the wgpu
  adapter PowerPreference (WGPU_POWER_PREF env still wins). Fixes the iGPU running
  hot under the shader background on hybrid laptops — pick the discrete GPU.
  New gpu_adapters() enumerates+dedups adapters; gpu_power_from_prefs /
  _options / _seed_index / _value_for_index.
- Ambient metaball added to the IMMERSIVE scene picker (mode 7, wgpu-only opt-in);
  the non-shader atmosphere stays the fallback for GL/software tiers.
- Glass treatment on the surfaces that were still opaque: carousel < > nav
  buttons (6 carousels), "Filter by category" pill, "Filter by genre" buttons,
  and the PlaylistBrowseView (View-all) header band + category radio row.
- i18n: "Preferred GPU" + description + discrete/integrated + restart toast (8
  locales).

Base: 76ea5a82. Prior checkpoint: a0f5cf2b.

cc

crates/qbz-ui/ui/discover/BrowseHeaderTools.slint

@@ -108,7 +108,7 @@ export component GenreButton inherits Rectangle {
     border-radius: 6px;
     background: root.active
         ? Theme.accent
-        : (ta.has-hover ? Theme.surface-hover : Theme.surface-elevated);
+        : (ta.has-hover ? Theme.surface-hover : (ShellState.app-background-active ? Theme.surface-elevated.with-alpha(AppearanceState.app-background-surface-alpha) : Theme.surface-elevated));
 
     genre-row := HorizontalLayout {
         padding-left: 12px;