Revert "feat: 侧边栏新增位置兼容模式，为 Windows 原生标题栏预留顶部空间"

This reverts commit ae71aecb83.

src/client/Sidebar.tsx

@@ -167,21 +167,6 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
     return () => { document.body.removeAttribute('data-dsh-sidebar-collapsed') }
   }, [collapsed])
 
-  // Position compatibility mode (titleBarCompat pref): Windows frameless
-  // windows draw the native title bar (minimize/maximize/close) at the
-  // window's top-right corner, OVER the web content. When the user enables
-  // the pref, the body attribute lets sidebar.module.css drop the toggle
-  // cluster below the strip and push the right panel's content below it.
-  // The attribute rides the snapshot's prefs, so flipping the setting
-  // re-renders and re-applies immediately; the cleanup removes it on
-  // unmount/boundary swap so a crashed sidebar never leaves it behind.
-  const titleBarCompat = snapshot.prefs.titleBarCompat
-  useEffect(() => {
-    if (titleBarCompat) document.body.setAttribute('data-dsh-title-bar-compat', '')
-    else document.body.removeAttribute('data-dsh-title-bar-compat')
-    return () => { document.body.removeAttribute('data-dsh-title-bar-compat') }
-  }, [titleBarCompat])
-
   /**
    * Bottom-panel merge on narrow viewports: whenever a session is current
    * while narrow (mount, session switch, or a desktop→narrow transition),