feat(plugins): show installed state in marketplace and search installed extensions

Marketplace cards now reflect whether an extension is already installed in
the current workspace, and the installed-extension list gains a search box.

Backend (stream install progress):
- _read_httpx_response_limited gains an optional task_context: it publishes
  download_total from Content-Length before the first chunk and updates
  download_current / download_speed per chunk. The marketplace download path
  previously had no progress reporting; it now matches the GitHub path.
- _marketplace_get forwards task_context to that helper.
- install_plugin resets the per-install counters so re-installing the same
  plugin cannot inherit stale metadata, and reports human-readable stages:
  preparing -> downloading -> inspecting -> storing -> installing
  dependencies -> launching -> waiting for plugin to become ready.

Frontend (installed state):
- New marketplace-installed helper normalises the sidebar identities
  (plugin: author/name, mcp: author__name, skill: bare name) into one
  type:author/name index and resolves a card's installed state from it.
  useMarketplaceInstalledIndex memoises on the sidebar lists, so a finished
  install (which refreshes the sidebar) re-evaluates the cards automatically.
- PluginMarketCardVO carries installed / hasUpdate. An installed extension
  turns its download affordance into a hollow green ring with a green check
  in place, instead of adding a separate badge; the count slot switches to
  the installed label. Cards with an available update use amber.
- PluginMarketComponent derives the annotated list and shares the index with
  RecommendationLists.

Frontend (install task UI):
- mapActionToStage matches the new connector stage strings. The pre-download
  stages are checked before the generic "install" match, because
  "preparing plugin install" also contains "install".
- Stage progress ranges are non-overlapping; overall progress interpolates on
  real byte counts while downloading and drifts monotonically elsewhere,
  capped at 99%.
- The progress dialog and task queue expose the launching stage.

Frontend (installed list search):
- The installed list had no search at all. A query box in the page header
  filters by label / name / author / description, case-insensitively, applied
  before grouping so grouped and flat views both honour it.
- Search misses and an empty list now show distinct empty states, with a
  clear action on a search miss.
- AsyncTask entity gains the optional created_at field.

i18n: new marketplace / install / search strings across all 8 locales.

Verified: ruff format + check, tsc --noEmit, prettier --check, eslint
(0 errors), and 89/89 frontend unit tests.

web/src/i18n/locales/ru-RU.ts

@@ -44,6 +44,7 @@ const ruRU = {
     delete: 'Удалить',
     add: 'Добавить',
     select: 'Выбрать',
+    clear: 'Очистить',
     skill: 'Навык',
     cancel: 'Отмена',
     submit: 'Отправить',
@@ -563,6 +564,8 @@ const ruRU = {
     getPluginListError: 'Не удалось получить список плагинов:',
     noPluginInstalled: 'Плагины не установлены',
     noExtensionInstalled: 'Расширения не установлены',
+    searchInstalled: 'Поиск установленных расширений',
+    noMatchingExtensions: 'Нет расширений, соответствующих «{{query}}»',
     loadingExtensions: 'Загрузка расширений...',
     groupByType: 'Группировать по формату',
     pluginConfig: 'Настройка плагина',
@@ -836,6 +839,9 @@ const ruRU = {
       noTags: 'Нет доступных тегов',
     },
     installCard: 'Установить {{name}}',
+    installedCard: '{{name}} установлен',
+    installed: 'Установлено',
+    updateAvailable: 'Доступно обновление',
   },
   mcp: {
     title: 'MCP',