feat(desktop): enable macOS version awareness for UI adjustments

- Fetch macOS major version from Rust to determine layout
- Adjust header padding and height for macOS versions

packages/ui/src/lib/desktop.ts

@@ -89,6 +89,7 @@ export type DesktopSettingsApi = {
 
 export type DesktopApi = {
   homeDirectory?: string;
+  macosMajorVersion?: number | null;
   getServerInfo: () => Promise<DesktopServerInfo>;
   restartOpenCode: () => Promise<{ success: boolean }>;
   shutdown: () => Promise<{ success: boolean }>;