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-primitives/src/WebBlock.module.css

@@ -91,6 +91,11 @@
   font: var(--dsw-font-xs-13);
 }
 
+.empty {
+  color: var(--dsw-alias-label-secondary);
+  font: var(--dsw-font-xs-13);
+}
+
 /* The fetch card is a compact summary: the URL over a status/truncation row. */
 .fetch {
   display: flex;