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/prefs-shared.ts

@@ -27,6 +27,13 @@ export interface SidebarPrefs {
    * user explicitly enables it in the side card settings.
    */
   agentTerminalTools: boolean
+  /**
+   * Whether expanding the bottom panel for the FIRST time in a session tries
+   * to open a fresh terminal tab there (the terminal quota/type still gates
+   * the attempt). On by default; the switch lives under the terminal tab's
+   * row in the Side card settings.
+   */
+  bottomPanelAutoTerminal: boolean
   /**
    * Whether chat-side file opens (tool-row path links, the produced-files
    * row, prose file mentions — every path that funnels through the client
@@ -97,6 +104,7 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   defaultWidthPercent: WIDTH_PERCENT_DEFAULT,
   autoOpenSubagent: true,
   agentTerminalTools: false,
+  bottomPanelAutoTerminal: true,
   interceptOpenPath: true,
   htmlViewerNoSandbox: false,
   htmlViewerDefaultUnsafe: false,