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。

tests/e2e/mount.e2e.ts

@@ -175,6 +175,26 @@ test('plugin mounts into the DSH shell and survives a built-in tab sweep', async
   const tabBar = sidebar.locator('[title]')
   await expect(tabBar.first()).toBeAttached({ timeout: 90_000 })
 
+  // Regression: with the panels still COLLAPSED, the document must not grow
+  // beyond the viewport. Collapsed panels are slid off-screen with
+  // `transform: translate(102%)`, and a transformed element still
+  // contributes to its ancestors' scrollable overflow — unclipped, the
+  // hidden panels extended the document's scroll area and the whole page
+  // became draggable left-right and up-down. The panel host clips at the
+  // viewport edge (overflow: hidden), so scrollWidth/scrollHeight must stay
+  // within the viewport here. (Assert <= not == : a classic scrollbar
+  // narrows clientWidth, so scrollWidth may sit slightly under innerWidth.)
+  await expect
+    .poll(
+      () =>
+        page.evaluate(() => ({
+          overflowX: document.documentElement.scrollWidth <= window.innerWidth,
+          overflowY: document.documentElement.scrollHeight <= window.innerHeight,
+        })),
+      { timeout: 30_000 },
+    )
+    .toEqual({ overflowX: true, overflowY: true })
+
   // openByDefault defaults OFF: a fresh session's panel starts collapsed.
   // Expand it through the toggle cluster before the layout push can apply.
   const expandButton = sidebar.getByRole('button', { name: 'Expand sidebar' })