fix(client-web): pin keyed WebRow in boot smoke, show empty-search feedback

- built-boot smoke asserts [data-variant=web][data-tool=web_search/fetch]
  (the keyed WebRow) instead of [data-web] (which WebBlock draws even on the
  GenericToolCard fallback, so a silent keyed-registration failure passed).
- WebBlock renders an explicit empty-state note when a search returns no
  answer and no sources, instead of a blank <ol>; the chat row does not
  surface the raw result content, so the backend's 'No results found.' was
  otherwise invisible.
- README (ui-conversation, ui-primitives) and the frontend Agent Note record
  the unknown-web-kind null arm, the details-panel flattened body, and the
  empty-search copy; pairings re-recorded.

packages/client/ui-conversation/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-conversation/README.md
-README.md: 7b39702d8828b80b50d4bc37dbc9350a674b7fdf
-README.zh.md: 7f3989e6446581f8264b6fa605dd975be8518ad7
+README.md: 84671fe067917f4aa38bccfaa413404652456fc2
+README.zh.md: 7aea71890bb725c7a87d9fb12804d9477f4eed7a