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/components/layout/Sidebar.tsx

@@ -127,7 +127,7 @@ export const Sidebar: React.FC<SidebarProps> = ({ isOpen, isMobile, children, cl
             ref={sidebarRef}
             className={cn(
                 'relative flex h-full overflow-hidden border-r border-border/40 will-change-[width] motion-reduce:transition-none',
-                'bg-sidebar',
+                'bg-sidebar oc-vibrancy-surface',
                 !isOpen && 'border-r-0',
                 className,
             )}