feat(web): surface Box disabled/unavailable state across consumers

When Box is disabled in config (``box.enabled = false``) or fails to
connect, every dependent UI surface now degrades visibly:

- ``useBoxStatus`` hook: shared, polled 30s, exposes ``available``,
  ``disabled`` (config-off) and a single ``hint`` key so callers don't
  have to re-derive the three states
- ``BoxUnavailableNotice`` reusable Alert banner driven by that hint
- Dashboard SystemStatusCards: three-state dot + label
  (connected / disabled-gray / disconnected-red); disabled state shows
  the ``boxDisabled`` hint, failed state continues to show the connector
  error. Plugin block kept untouched
- Skills page (create view) and SkillDetailContent (edit view):
  Save button disabled and banner inserted above the form when Box is
  unavailable — matches the backend gate added in the previous commit
- PipelineExtension skill section: ``enable_all_skills`` switch, Add
  Skill button and Remove buttons all gate on Box availability;
  banner inline under the section header
- PipelineFormComponent: banner above the ``local-agent`` stage card
  when Box is unavailable, since that stage carries the sandbox-bound
  ``box-session-id-template`` field
- Box status payload type (``ApiRespBoxStatus.enabled``) and 8 locale
  files updated with ``boxDisabled`` / ``boxUnavailable`` /
  ``boxRequiredHint`` strings

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -1311,6 +1311,12 @@ const ruRU = {
     disabled: 'Отключено',
     statusDetail: 'Статус',
     pluginDisabled: 'Система плагинов отключена',
+    boxDisabled:
+      'Песочница Box отключена в конфигурации — инструменты песочницы, добавление/редактирование навыков и stdio MCP недоступны',
+    boxUnavailable:
+      'Песочница Box недоступна — инструменты песочницы, добавление/редактирование навыков и stdio MCP недоступны',
+    boxRequiredHint:
+      'Для этой функции требуется среда Box. Включите её в конфигурации (box.enabled = true) и убедитесь, что соединение работает.',
     boxBackend: 'Бэкенд',
     boxProfile: 'Профиль',
     boxSandboxes: 'Песочницы',