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/state.slint

@@ -3215,6 +3215,10 @@ export global AppearanceState {
     in-out property <bool> renderer-setting-visible: false;
     in-out property <[string]> renderer-options: [@tr("Auto (recommended)"), @tr("GPU"), @tr("GPU (compatibility)"), @tr("Software")];
     in-out property <int> renderer-index: 0;
+    // Preferred GPU: options built in Rust from the detected adapters
+    // ("Auto" + each GPU by name); index 0 = Auto.
+    in-out property <[string]> gpu-power-modes: [@tr("Auto (recommended)")];
+    in-out property <int> gpu-power-index: 0;
 
     // --- INTERFACE SIZE ----------------------------------------------------
     // Window scale-factor preset (XS 0.8 / Small 0.9 / Default 1.0 /