feat(mcp): add standalone server registry

Implement C.1 stdio MCP server CRUD, encrypted environment secrets, command digest approval, connection tests, lifecycle recovery, and dynamic tool registration. Add the standalone frontend configuration center, contracts, regression tests, and development documentation.

frontend/src/components/common/PrimarySidebar.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { useRoute, useRouter } from 'vue-router'
 import { computed, ref } from 'vue'
-import { ArrowLeftBold, ArrowRightBold, Brush, ChatDotRound, CircleCheck, Connection, Cpu, FolderOpened, Lightning, Search, Setting } from '@element-plus/icons-vue'
+import { ArrowLeftBold, ArrowRightBold, Brush, ChatDotRound, CircleCheck, Connection, Cpu, FolderOpened, Lightning, Monitor, Search, Setting } from '@element-plus/icons-vue'
 import AppIcon from './AppIcon.vue'
 
 const route = useRoute()
@@ -16,6 +16,7 @@ const navItems = [
   { name: 'tasks', icon: CircleCheck, label: '任务' },
   { name: 'skills', icon: Lightning, label: 'Skill' },
   { name: 'plugins', icon: Connection, label: 'Plugin' },
+  { name: 'mcp-servers', icon: Monitor, label: 'MCP' },
   { name: 'themes', icon: Brush, label: '主题' },
   { name: 'settings', icon: Setting, label: '设置' },
 ]