feat(ui): localize friendly agent errors and surface them as toasts

Thread a translate function through friendlyAgentErrorMessage/
friendlyProjectFailureMessage and move the hard-coded zh-CN error copy
into the i18n locale files (with a zh-CN fallback map). Generation
stream failures now report through a toast via showGenerationError in
addition to setError, so the removed inline .panel-error footer is
gone.

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

frontend/src/ui/components/CanvasAgentTimeline/index.tsx

@@ -459,11 +459,12 @@ function parseResearchPayload(message: AgentMessage): AgentResearchPayload | nul
 }
 
 function MessageBubble({ message }: { message: AgentMessage }) {
+  const { t } = useI18n();
   const [copied, setCopied] = useState(false);
   const [copyButtonVisible, setCopyButtonVisible] = useState(false);
   const hideCopyButtonTimerRef = useRef<number | null>(null);
   const rawContent = cleanMessageContent(message.content);
-  const friendlyError = message.role === "error" ? friendlyAgentErrorMessage({ ...message, content: rawContent }) : null;
+  const friendlyError = message.role === "error" ? friendlyAgentErrorMessage({ ...message, content: rawContent }, t) : null;
   const content = friendlyError?.content || rawContent;
   const title = friendlyError?.title || message.title;