perf: reduce streaming render cost and refresh activity page on demand

- Segment streaming Markdown so completed blocks are reused and only the growing tail is re-parsed; fall back to whole-document parsing for document-wide constructs
- Memoize conversation sidebar rows and cache per-conversation derived activity, ordering and pending approvals
- Activity page loads on entry, refreshes every 15 s while visible, and has a single header refresh button
- Include pending settings, model download source, device sharing and knowledge gateway changes in the working tree
- Add Electron streaming input benchmark and regression tests

docs/features/application-tool-navigation/ui-design.md

@@ -92,8 +92,9 @@
 
 ## 4. 应用中心与共同设置
 
-设备共享追加到默认顺序末尾。管理行显示“内置”、打开与排序按钮，不提供通用启用或常驻
-开关；应用菜单始终保留该项，点击后在主内容区打开[设备共享页面](../device-sharing/ui-design.md#当前首期页面)。
+设备共享追加到默认顺序末尾，测试阶段默认关闭。管理行显示“可选”及启用状态，提供打开、
+设置与排序按钮；设置内提供启用 Switch，不提供常驻开关。关闭时禁用“打开”，应用菜单只在
+启用时显示该项，点击后在主内容区打开[设备共享页面](../device-sharing/ui-design.md#当前首期页面)。
 
 底部点击打开轻量菜单，锚定按钮向上展开，无模态遮罩、背景隔离或焦点循环。菜单按共享保存顺序列出已启用应用，不按打开历史或常驻状态过滤。点击应用行关闭菜单，本机推理监控打开独立 Modal，知识库、智能心跳和魔法笔记在主内容区打开页面。分隔线下的“管理应用”关闭菜单并打开既有管理 Modal。