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.module.css

@@ -209,28 +209,6 @@
   padding-right: 40px;
 }
 
-/* ── Position compatibility mode (Windows native title bar) ────────────── */
-
-/* Windows frameless / hidden-title-bar windows draw the native caption
-   strip (minimize / maximize / close) at the window's top-right corner,
-   OVER the web content. When the user enables the "position compatibility"
-   pref, the sidebar yields the strip: the toggle cluster drops below it and
-   the right panel's content starts below it — the panel's surface still
-   spans the full height, so the caption buttons float over empty sidebar
-   fill (the VSCode hidden-title-bar look). The bottom panel is unaffected:
-   it sits at the window's bottom, never under the top strip. The Sidebar
-   shell toggles the body attribute from the prefs. */
-:global(body[data-dsh-title-bar-compat]) .toggleCluster {
-  top: 43px; /* 40px caption strip + the original 3px inset */
-}
-
-:global(body[data-dsh-title-bar-compat]) .panel {
-  /* The caption strip is drawn over the panel's own top edge: keep the
-     surface full-height and push the content (the tab strip) below the
-     strip instead. .panelResize is absolutely positioned and unaffected. */
-  padding-top: 40px;
-}
-
 /* The shared corner (both panels open): the intersection of the right
    panel's left edge and the bottom panel's top edge. Horizontal drags
    resize the width, vertical drags the height — the two panels drag