refactor: remove macOS vibrancy support and unify glass surfaces

Drops the macOS vibrancy toggle, IPC, and related settings copy
Updates dialogs, popovers, tooltips, and dropdowns to use shared glass styles
Adds sticky header fade behavior to model picker and sidebar lists

packages/ui/src/components/layout/Header.tsx

@@ -336,7 +336,7 @@ const DesktopServicesMenu = React.memo(function DesktopServicesMenu({
       </Tooltip>
       <DropdownMenuContent
         align="end"
-        className="w-[min(27rem,calc(100vw-2rem))] max-h-[75vh] overflow-y-auto bg-[var(--surface-elevated)] p-0"
+        className="w-[min(27rem,calc(100vw-2rem))] max-h-[75vh] overflow-y-auto p-0"
       >
         {isDesktopApp ? (
           <div>

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 will-change-[width] motion-reduce:transition-none',
-                'bg-sidebar oc-vibrancy-surface',
+                'bg-sidebar',
                 !isOpen && 'border-r-0',
                 className,
             )}

packages/ui/src/components/layout/TitlebarLeftControls.tsx

@@ -28,7 +28,6 @@ const ICON_BUTTON_CLASS =
 export const TitlebarLeftControls: React.FC = () => {
   const { t } = useI18n();
   const toggleSidebar = useUIStore((state) => state.toggleSidebar);
-  const isSidebarOpen = useUIStore((state) => state.isSidebarOpen);
   const shortcutOverrides = useUIStore((state) => state.shortcutOverrides);
   const projectActionsContext = useProjectActionsContext();
   const clusterRef = React.useRef<HTMLDivElement | null>(null);
@@ -129,10 +128,6 @@ export const TitlebarLeftControls: React.FC = () => {
           <ProjectActionsButton
             projectRef={projectActionsContext.projectRef}
             directory={projectActionsContext.directory}
-            // While the sidebar is open the controls sit over the frosted
-            // sidebar — let the pill share its translucency instead of painting
-            // an opaque surface (handled under [data-oc-vibrancy] in CSS).
-            className={isSidebarOpen ? 'oc-vibrancy-pill' : undefined}
           />
         ) : null}
       </div>