test(web): refresh the Markdown image golden for disabled non-tail forks

PR #1003 re-recorded this golden from a base that predated PR #1150, which
disables the branch control on any message that is not a completed turn's
tail. Both landed on master, whose CI skips the snapshot lane, so the
conflict only surfaces on PRs. The first message's control is now disabled
with its explanatory text.

apps/web/tests/snapshots/markdown-images/ui.expected.md

@@ -7,8 +7,9 @@
 - text: Show the Markdown image policy. {{clock}}
 - button "Copy":
   - img
-- button "Branch into a new conversation":
+- button "Branch into a new conversation" [disabled]:
   - img
+- text: Available only on the last message of a completed turn
 - heading "Markdown images" [level=2]
 - paragraph:
   - img "Remote test image"