feat(feedback): add the Web surface for message feedback

Consume the durable message-feedback sidecar from #2217 in the browser:
per-message Like/Dislike with an optional note, contributed through a
declared assistant-actions slot.

- carry MessageId on finalized AssistantMessageNode so a target is nameable
- declare conversation.chat.assistant-actions and render it in the
  IconActions row between copy and branch
- hold one FeedbackController per Session with per-item ifVersion CAS,
  reconciling a version-conflict from the reply's authoritative item
- mount messageFeedbackRemote alongside goalsRemote

apps/web/tests/snapshots/code-mode-round/ui.expected.md

@@ -33,6 +33,10 @@
 - paragraph: DONE
 - button "Copy":
   - img
+- button "Good response":
+  - img
+- button "Bad response":
+  - img
 - button "Branch into a new conversation":
   - img
 - text: {{clock}} Ran for {{duration}} TTFT {{duration}} {{throughput}} tok/s