feat(desktop): macOS vibrancy for the left sidebar with a toggle

Add native macOS vibrancy behind the left sidebar (the only translucent
surface; header/chat/right sidebar stay opaque), plus a setting to turn it off.

- Window created with vibrancy applied after first show (avoids the cold-launch
  no-composite quirk); minimize/restore suppress the frost during the genie
  animation. Renderer frosts the sidebar via --sidebar-vibrancy-overlay once
  data-oc-vibrancy[-ready] are set; project-actions pill matches when open.
- data-oc-vibrancy-ready defaults are set in cssGenerator (DOM guaranteed),
  not the preload (document-start race left the sidebar un-frosted on launch).
- prefers-reduced-transparency falls back to solid surfaces.
- Appearance settings (macOS desktop only): a checkbox to enable/disable
  vibrancy, persisted to settings.json and applied via a Save & restart button
  (vibrancy is a window-creation option, so it needs a relaunch).

packages/ui/src/styles/design-system.css

@@ -142,6 +142,13 @@
     background-color: var(--background) !important;
   }
 
+  :root.desktop-runtime[data-oc-vibrancy],
+  :root.desktop-runtime[data-oc-vibrancy] body,
+  :root.desktop-runtime[data-oc-vibrancy] #root {
+    background: transparent !important;
+    background-color: transparent !important;
+  }
+
   .font-sans {
     font-family: var(--font-sans, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif) !important;
   }