feat(plugins): show installed state in marketplace and stream install tasks

- report download progress (bytes, speed) and human-readable install stages
  from the plugin runtime connector via the install task context
- add a marketplace installed-state helper so market cards reflect whether a
  plugin is already installed
- surface install progress / queue state in the plugin install-task UI
- extend BackendClient and API entities with install-task status
- i18n for the new marketplace / install strings

web/src/i18n/locales/zh-Hans.ts

@@ -715,6 +715,9 @@ const zhHans = {
     installConfirm: '确定要安装插件 "{{name}}" ({{version}}) 吗？',
     downloadComplete: '插件 "{{name}}" 下载完成',
     installFailed: '安装失败，请稍后重试',
+    installed: '已安装',
+    updateAvailable: '有更新',
+    alreadyInstalled: '{{name}} 已安装',
     loadFailed: '获取插件列表失败，请稍后重试',
     noDescription: '暂无描述',
     recommendation: {