test(web): run the subagent-conversation e2e against the locale-aware copy

The ui-subagent catalog and read-only composer copy moved from hardcoded
Chinese to the locale-aware `subagent` namespace, so an en-US headless
browser now renders English. The e2e's selectors and goldens still
asserted the old hardcoded Chinese strings, leaving the scenario unable
to find the catalog trigger.

Convert the selectors to the default (en-US) render and re-record the
catalog goldens (ui, tree, nested) in English. The locale-aware parts of
the remaining goldens were already English (recorded under the en-US
default), so sidebar and fork are untouched.

apps/web/tests/snapshots/subagent-conversation/nested.expected.md

@@ -14,5 +14,5 @@
 - button "Branch into a new conversation":
   - img
 - status:
-  - strong: 此子代理暂时只读
-  - text: 父会话当前不在线，重新打开父会话后即可继续发送消息。
+  - strong: This subagent is read-only for now
+  - text: The parent session is offline; reopen it to continue sending messages.