feat: 侧边栏双面板 — 右上角按钮簇、底部面板（首展自动终端/关闭按钮）、跨面板拖 Tab、拖动性能优化

- 右上角持久按钮簇：横向排列、独立 glyph（底栏/侧拉）、始终钉在视口右上角（侧边栏展开时位于其标签条右端，真实挤占 Tab 空间）
- 底部面板：独立工作台，只挤占中间 Agent 输出区（不覆盖左右侧边栏），右缘分割线、关闭 x、首展自动开终端（二级设置开关，默认开）
- 两面板拐角双向拖动调节尺寸；拖动改为 rAF 节流直写 DOM，消除卡顿；面板尺寸互钳制防丢失
- 跨面板拖 Tab（合并/拆分落点），moveTab/moveTabToEdge 支持跨树移动
- 欢迎卡片改响应式网格；布局/几何全部按会话持久化，旧状态平滑迁移

src/client/split-pane.tsx

@@ -262,20 +262,24 @@ function NodeView(props: {
   )
 }
 
-/** The workbench: the split tree filling the sidebar body. */
+/** The workbench: the split tree filling the sidebar body. `tree` selects
+ *  which tree renders (the right panel's by default; the bottom panel passes
+ *  `state.bottomSplits` — the actions route by pane id, so one action set
+ *  serves both). */
 export function Workbench(props: {
   state: SidebarState
+  tree?: SplitNode
   newTabOptions: NewTabOption[]
   actions: WorkbenchActions
   onNewTab: (optionId: string) => void
   renderTab: (tab: SidebarTab, active: boolean, paneId: string) => ReactNode
   getTabIcon?: (tab: SidebarTab) => ReactNode
 }) {
-  const { state, newTabOptions, actions, onNewTab, renderTab, getTabIcon } = props
+  const { state, tree, newTabOptions, actions, onNewTab, renderTab, getTabIcon } = props
   return (
     <div className={css.workbench}>
       <NodeView
-        node={state.splits}
+        node={tree ?? state.splits}
         state={state}
         newTabOptions={newTabOptions}
         actions={actions}