fix: Apply titleBarStyle only on macOS

titleBarStyle: 'hiddenInset' is a macOS-only option. Use conditional
spread to only apply it when process.platform === 'darwin', ensuring
Windows and Linux get consistent default styling.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/electron/windows/main-window.ts

@@ -46,7 +46,8 @@ export function createWindow(): void {
       contextIsolation: true,
       nodeIntegration: false,
     },
-    titleBarStyle: 'hiddenInset',
+    // titleBarStyle is macOS-only; use hiddenInset for native look on macOS
+    ...(process.platform === 'darwin' && { titleBarStyle: 'hiddenInset' as const }),
     backgroundColor: '#0a0a0a',
   };