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/contexts/ThemeSystemContext.tsx

@@ -345,7 +345,9 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
     if (typeof document === 'undefined') {
       return;
     }
-    const chromeColor = theme.colors.surface.background;
+    const hasMacVibrancy = document.documentElement.hasAttribute('data-oc-vibrancy')
+      || window.__OPENCHAMBER_ELECTRON__?.macVibrancy === true;
+    const chromeColor = hasMacVibrancy ? 'transparent' : theme.colors.surface.background;
 
     document.body.style.backgroundColor = chromeColor;