fix: 适配 DSH 0.1.x 的 [data-slot] DOM 结构，恢复 Session log 挤压与布局

DSH 0.1.x 把 slot 宿主包进 [data-slot] 容器：
#root > div[data-slot=root] > div.frame > div.centerCol
  > div[data-slot=conversation] > div.root[data-phase]
    > div[data-slot=conversation.session.header] > header

旧选择器 #root > div > div:nth-child(2) 选中的是 data-slot=root 包装
层的第 2 子元素（不存在，其唯一子元素是 frame），导致：
- 折叠态 header 右 padding 挤压失效（Session log 胶囊被按钮簇盖住）
- 底部面板纵向挤压 / 拖动过渡 / 减少动效全部失效
- Sidebar.tsx 中间列定位器（#root > div 的 children[1]）失效，
  底部面板宽度测量回退为 0

修复：全部改用 [data-slot] 语义锚点——
- 中间列：#root > div[data-slot=root] > div > div:nth-child(2)
- header：body[data-dsh-sidebar-collapsed]
  [data-slot=conversation.session.header] > header
- Sidebar.tsx 定位器：#root [data-slot=conversation] 的 parentElement
  （= centerCol，探针实测确认）

v0.10.1

src/client/layout.css

@@ -6,11 +6,13 @@
  * conversation output and the input bar, like a VSCode sidebar.
  *
  * The bottom panel squeezes ONLY the center column — it must not cover the
- * app's own left sidebar or the right panel. The AppFrame's grid items are
- * (#root > div > div:nth-child(2)): sidebarCol, centerCol, detailsCol, so
- * the vertical push lands on the center column alone. The fixed panels
- * cover the vacated strips, which looks seamless because #root's background
- * is the theme base.
+ * app's own left sidebar or the right panel. DSH 0.1.x wraps slot hosts in
+ * [data-slot] containers, so the AppFrame grid lives one level deeper:
+ * #root > div[data-slot="root"] > div (the frame). Its grid items are
+ * (frame > div:nth-child(1..3)): sidebarCol, centerCol, detailsCol, so the
+ * vertical push lands on the center column alone. The fixed panels cover
+ * the vacated strips, which looks seamless because #root's background is
+ * the theme base.
  *
  * The sizes ride CSS variables updated by the Sidebar shell (0 while
  * collapsed); expand/collapse animates both the margins and the panel
@@ -23,10 +25,10 @@
 }
 
 /* The AppFrame's grid items are sidebarCol, centerCol, detailsCol (children
-   1-3 of #root > div) — nth-child(2) is the center column. A stretched grid
-   item shrinks by its margins, so the conversation content (output + input
-   bar) lifts without touching the sidebars. */
-#root > div > div:nth-child(2) {
+   1-3 of #root > div[data-slot="root"] > div) — nth-child(2) is the center
+   column. A stretched grid item shrinks by its margins, so the conversation
+   content (output + input bar) lifts without touching the sidebars. */
+#root > div[data-slot="root"] > div > div:nth-child(2) {
   margin-bottom: var(--dsh-sidebar-height, 0px);
   transition: margin-bottom var(--ds-transition-duration-slow) var(--ds-ease-in-out);
 }
@@ -35,20 +37,23 @@
    corner. Push the DSH session header's right padding out so its right-aligned
    utilities (the "Session log" download capsule) yield the corner instead of
    hiding under the cluster. The header default right-pads 28px; the 2-button
-   cluster spans right 10→70px, so 78px clears it with an 8px gap. The
-   Sidebar shell toggles the body attribute with the panel open state. */
-body[data-dsh-sidebar-collapsed] #root > div > div:nth-child(2) header {
+   cluster spans right 10→70px, so 78px clears it with an 8px gap. Anchor on
+   the header's slot host wrapper ([data-slot="conversation.session.header"])
+   rather than a positional path: DSH 0.1.x nests the header several levels
+   under the center column. The Sidebar shell toggles the body attribute with
+   the panel open state. */
+body[data-dsh-sidebar-collapsed] [data-slot="conversation.session.header"] > header {
   padding-right: 78px;
 }
 
 body[data-dsh-sidebar-dragging] #root,
-body[data-dsh-sidebar-dragging] #root > div > div:nth-child(2) {
+body[data-dsh-sidebar-dragging] #root > div[data-slot="root"] > div > div:nth-child(2) {
   transition: none;
 }
 
 @media (prefers-reduced-motion: reduce) {
   #root,
-  #root > div > div:nth-child(2) {
+  #root > div[data-slot="root"] > div > div:nth-child(2) {
     transition: none;
   }
 }