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/components/layout/Header.tsx

@@ -108,15 +108,28 @@ export const Header: React.FC = () => {
   }, []);
 
   const macosMajorVersion = React.useMemo(() => {
+    if (typeof window === 'undefined') {
+      return null;
+    }
+    // Use Tauri-provided version if available (accurate), otherwise fall back to UA parsing
+    const desktopApi = (window as typeof window & { opencodeDesktop?: { macosMajorVersion?: number | null } }).opencodeDesktop;
+    if (desktopApi?.macosMajorVersion != null) {
+      return desktopApi.macosMajorVersion;
+    }
+    // Fallback: WebKit reports "Mac OS X 10_15_7" format where 10 is legacy prefix
     if (typeof navigator === 'undefined') {
       return null;
     }
-    const match = (navigator.userAgent || '').match(/Mac OS X (\d+)[._]/);
+    const match = (navigator.userAgent || '').match(/Mac OS X (\d+)[._](\d+)/);
     if (!match) {
       return null;
     }
-    const parsed = Number.parseInt(match[1], 10);
-    return Number.isNaN(parsed) ? null : parsed;
+    const first = Number.parseInt(match[1], 10);
+    const second = Number.parseInt(match[2], 10);
+    if (Number.isNaN(first)) {
+      return null;
+    }
+    return first === 10 ? second : first;
   }, []);
 
   useEffect(() => {
@@ -296,19 +309,22 @@ export const Header: React.FC = () => {
   const desktopPaddingClass = React.useMemo(() => {
     if (isDesktopApp && isMacPlatform) {
       // Always reserve space for Mac traffic lights since header is always on top
-      return 'pl-[5.125rem]';
+      return 'pl-[5.5rem]';
     }
     return 'pl-3';
   }, [isDesktopApp, isMacPlatform]);
 
   const macosHeaderSizeClass = React.useMemo(() => {
-    if (!isDesktopApp || !isMacPlatform) {
+    if (!isDesktopApp || !isMacPlatform || macosMajorVersion === null) {
       return '';
     }
-    if (macosMajorVersion === null || macosMajorVersion > 15) {
-      return '';
+    if (macosMajorVersion >= 26) {
+      return 'h-12';
     }
-    return 'h-14';
+    if (macosMajorVersion <= 15) {
+      return 'h-14';
+    }
+    return '';
   }, [isDesktopApp, isMacPlatform, macosMajorVersion]);
 
   const updateHeaderHeight = React.useCallback(() => {
@@ -351,7 +367,7 @@ export const Header: React.FC = () => {
 
   useEffect(() => {
     updateHeaderHeight();
-  }, [updateHeaderHeight, isMobile]);
+  }, [updateHeaderHeight, isMobile, macosHeaderSizeClass]);
 
   const handleDragStart = React.useCallback(async (e: React.MouseEvent) => {
     if ((e.target as HTMLElement).closest('button, a, input, select, textarea')) {