活动栏移除主题按钮：低频操作收纳进 设置 → 外观

设置弹窗已有 外观 → 界面主题（深色/浅色）选项，活动栏不再重复占位；
底部精简为 文档 / 设置 / 账号

packages/ui/src/components/ActivityRail.tsx

@@ -18,8 +18,6 @@ export function ActivityRail() {
   const authLogout = useWorkspace((s) => s.authLogout);
   const syncNow = useWorkspace((s) => s.syncNow);
   const setDialog = useWorkspace((s) => s.setDialog);
-  const theme = useWorkspace((s) => s.settings.theme);
-  const setTheme = useWorkspace((s) => s.setTheme);
 
   const [spaceMenu, setSpaceMenu] = useState<{ x: number; y: number } | null>(null);
   const [accountMenu, setAccountMenu] = useState<{ x: number; y: number } | null>(null);
@@ -195,19 +193,11 @@ export function ActivityRail() {
 
       <div className="flex-1" />
 
-      {/* 底部：文档 / 主题 / 设置 / 账号 */}
+      {/* 底部：文档 / 设置 / 账号（主题切换在 设置 → 外观，低频操作不占栏位） */}
       <a className="rail-item" href="/docs" target="_blank" rel="noopener noreferrer" title="用户文档">
         <Icon name="book-open" size={16} />
         <span className="text-[10px]">文档</span>
       </a>
-      <button
-        className="rail-item"
-        title={theme === "dark" ? "切换到浅色主题" : "切换到深色主题"}
-        onClick={() => setTheme(theme === "dark" ? "light" : "dark")}
-      >
-        <Icon name={theme === "dark" ? "sun" : "moon"} size={16} />
-        <span className="text-[10px]">主题</span>
-      </button>
       <button className="rail-item" title="设置" onClick={() => setDialog({ type: "settings" })}>
         <Icon name="settings" size={16} />
         <span className="text-[10px]">设置</span>