feat(mcp-web): block stdio MCP creation at the form when Box is unavailable

When Box is disabled in config (``box.enabled = false``) or unreachable,
saving a new MCP server in stdio mode produced one that could never
start — the user would only learn that from the runtime error on the
detail page. Stop the user before they save instead.

Both MCP forms (the page-level ``MCPForm.tsx`` and the older dialog
``MCPFormDialog.tsx``) now:

- Disable the ``stdio`` option in the mode select when Box is
  unavailable, with a small "(requires Box)" suffix so the reason is
  obvious. Existing stdio configs still display their current value
- Show ``BoxUnavailableNotice`` inline under the mode select when the
  currently-selected mode is stdio and Box is unavailable, so editing
  a stale stdio config makes the cause visible
- Disable the Save / Submit button while stdio is selected under that
  condition. ``MCPForm`` exposes a new ``onSaveBlockedChange`` prop
  so the parent ``MCPDetailContent`` can disable both its Submit and
  Save buttons. ``MCPFormDialog`` disables its Save button locally
- Refuse the submit handler too (Enter-key path) with a toast carrying
  the same i18n message

i18n: ``mcp.boxRequired`` (short tag in the disabled option) and
``mcp.stdioBlockedByBoxToast`` added to all 8 locales.

Backend runtime gate (``_init_stdio_python_server`` refusal +
``BOX_UNAVAILABLE`` error_phase + retry short-circuit) stays in place
as the last line of defence for API bypass.

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

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

@@ -716,6 +716,9 @@ const zhHans = {
       'Stdio 模式的 MCP 服务器依赖 Box 沙箱，目前无法连接。',
     boxStdioRefusedSuggestion:
       '请启用 Box（box.enabled = true）并确认运行时连接正常，或将此服务器切换到 http/sse 模式。',
+    boxRequired: '需要 Box',
+    stdioBlockedByBoxToast:
+      'Box 沙箱已禁用或不可用，无法保存 stdio 模式的 MCP。请启用 Box 或改为 http/sse 模式。',
     toolsFound: '个工具',
     unknownError: '未知错误',
     noToolsFound: '未找到任何工具',