feat: 位置兼容模式支持自定义下移距离（齿轮弹窗数字输入）

titleBarStripPx 新偏好（0–120px，默认 40）：侧边栏按钮与内容的下移量改为
CSS 变量 --dsh-title-bar-strip 驱动，Sidebar shell 随 prefs 写入。设置页
「常规」的位置兼容模式行在开启时显示齿轮小图标，弹窗内用数字行设置下移
距离（复用 FeatureSettingsRows 行控件与提交钳制）。

src/client/sidebar.module.css

@@ -217,18 +217,19 @@
    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. */
+   fill (the VSCode hidden-title-bar look). The strip height is user-tunable
+   (`--dsh-title-bar-strip`, written by the Sidebar shell from
+   `titleBarStripPx`; 40px fallback). The bottom panel is unaffected: it
+   sits at the window's bottom, never under the top strip. */
 :global(body[data-dsh-title-bar-compat]) .toggleCluster {
-  top: 43px; /* 40px caption strip + the original 3px inset */
+  top: calc(var(--dsh-title-bar-strip, 40px) + 3px);
 }
 
 :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;
+  padding-top: var(--dsh-title-bar-strip, 40px);
 }
 
 /* The shared corner (both panels open): the intersection of the right