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/session/sidebar/SessionNodeItem.tsx

@@ -1143,7 +1143,6 @@ function SessionNodeItemComponent(props: Props): React.ReactNode {
           data-slot="dropdown-menu-content"
           finalFocus={() => (renamingFolderId || editingIdRef.current) ? false : true}
           style={{
-            backgroundColor: 'var(--surface-elevated)',
             color: 'var(--surface-elevated-foreground)',
           }}
           className={cn(dropdownMenuPopupClass, 'min-w-[180px]')}

packages/ui/src/components/session/sidebar/SidebarProjectsList.tsx

@@ -218,7 +218,7 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
       hideTopScrollShadow={!enableStickyFade}
       scrollShadowSize={96}
       outerClassName="flex-1 min-h-0"
-      className={cn('oc-sidebar-scroller space-y-1.5 pb-1 pl-2.5 pr-2 [overflow-anchor:none]', props.mobileVariant ? '' : '')}
+      className={cn('oc-sidebar-scroller oc-sticky-fade-scroller space-y-1.5 pb-1 pl-2.5 pr-2 [overflow-anchor:none]', props.mobileVariant ? '' : '')}
       style={enableStickyFade ? { '--scroll-shadow-top-size': '0px' } as React.CSSProperties : undefined}
       onScroll={enableStickyFade ? (event) => syncTopFade(event.currentTarget) : undefined}
     >