feat(canvas): show loading state while agent history loads

Track agent history loading in the workspace and render a spinner with a
localized "loading conversation history" message in the assistant feed,
so the empty state no longer flashes before history resolves.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -341,6 +341,7 @@
   "conversation": "Chat",
   "newChat": "New chat",
   "conversationHistory": "History",
+  "loadingConversationHistory": "Loading conversation history...",
   "scrollToLatest": "Scroll to latest",
   "searchConversationPlaceholder": "Search conversations",
   "deleteConversation": "Delete conversation",