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

新增 SidebarPrefs.titleBarCompat 设置项（默认关闭）：开启后 toggle cluster
下移 40px、右侧面板内容从标题栏条带下方开始，避免被 Windows 无边框窗口
右上角的原生标题栏遮挡。底部面板不受影响。设置页「常规」新增一行开关，
zh/en 双语，读写下穿既有 settings 路由。测试覆盖 parsePrefs 回退、schema
默认值、设置页渲染与开关状态。

src/client/sidebar.module.css

@@ -209,6 +209,28 @@
   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