fix(layout): 面板宿主裁剪视口边缘，收起的面板不再撑出文档双向滚动 (#277)

收起的面板以 transform: translate(102%) 滑出屏幕，但 transform 后的元素仍计入祖先滚动溢出区，导致整个页面可左右/上下拖拽。面板宿主 [data-dsh-panel-host] 是视口尺寸的 fixed 容器，补上 overflow: hidden 后裁剪点恰在视口边缘，开合滑入动画、内部滚动容器与 fixed 弹层均不受影响。e2e 增加回归断言：面板收起时文档 scrollWidth/Height 不得超出视口。

src/client/sidebar.module.css

@@ -34,6 +34,15 @@
   inset: 0;
   z-index: 40;
   pointer-events: none;
+  /* Clip at the viewport edge: collapsed panels slide off-screen with
+     `transform: translate(102%)`, and a transformed element still
+     contributes to its ancestors' scrollable overflow — without this the
+     hidden panels extended the document's scrollable area, making the whole
+     page draggable left-right and up-down while any panel was collapsed.
+     The host is exactly viewport-sized (inset: 0), so the clip is invisible
+     until a panel slides in from the edge; internal scroll containers and
+     the app's fixed overlay stack are unaffected. */
+  overflow: hidden;
 }
 
 :global([data-dsh-panel-host][data-dsh-panel-host-degraded]) {