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/contexts/ThemeSystemContext.tsx

@@ -347,9 +347,7 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
     if (typeof document === 'undefined') {
       return;
     }
-    const hasMacVibrancy = document.documentElement.hasAttribute('data-oc-vibrancy')
-      || window.__OPENCHAMBER_ELECTRON__?.macVibrancy === true;
-    const chromeColor = hasMacVibrancy ? 'transparent' : theme.colors.surface.background;
+    const chromeColor = theme.colors.surface.background;
 
     document.body.style.backgroundColor = chromeColor;