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/builtins/tabs.tsx

@@ -106,14 +106,19 @@ export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
       icon: (size: number) => <IconTerminalOutline16 size={size} />,
       order: 40,
       available: (_ctx, _scope, state) => uiTerminalCount(state) < TERMINAL_LIMIT,
-      // Declarative settings: the model-facing terminal tools switch renders
-      // under this row in the Side card settings page (the Terminal page's
-      // own related setting; the host gates the toolset on it independently).
+      // Declarative settings: the model-facing terminal tools switch and the
+      // bottom-panel first-expansion auto-terminal switch render under this
+      // row in the Side card settings page (the Terminal page's own related
+      // settings; the host gates the toolset on the tools one independently).
       settings: {
         toggles: [{
           key: 'agentTerminalTools',
           title: () => t('settingsToolsTitle'),
           desc: () => t('settingsToolsDesc'),
+        }, {
+          key: 'bottomPanelAutoTerminal',
+          title: () => t('settingsBottomTerminalTitle'),
+          desc: () => t('settingsBottomTerminalDesc'),
         }],
       },
       createTab: (state) => {