fix(frontend): citations display + refactor link/citation utils

- Citations: no underline while streaming (message links); artifact markdown external links as citation cards
- Refactor: add isExternalUrl, syntheticCitationFromLink in core/citations; shared externalLinkClass in lib/utils; simplify message-list-item and artifact-file-detail link rendering

修复引用展示并抽离链接/引用工具
- 引用：流式输出时链接不这下划线；Artifact 内 Markdown 外链以引用卡片展示
- 重构：core/citations 新增 isExternalUrl、syntheticCitationFromLink；lib/utils 共享 externalLinkClass；精简消息与 Artifact 中的链接渲染逻辑

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/core/citations/index.ts

@@ -1,9 +1,11 @@
 export {
-  parseCitations,
   buildCitationMap,
   extractDomainFromUrl,
   isCitationsBlockIncomplete,
+  isExternalUrl,
+  parseCitations,
   removeAllCitations,
+  syntheticCitationFromLink,
 } from "./utils";
 
 export type { Citation, ParseCitationsResult } from "./utils";

frontend/src/core/citations/utils.ts

@@ -118,6 +118,25 @@ export function buildCitationMap(
   return map;
 }
 
+/**
+ * Whether the URL is external (http/https).
+ */
+export function isExternalUrl(url: string): boolean {
+  return url.startsWith("http://") || url.startsWith("https://");
+}
+
+/**
+ * Build a synthetic Citation from a link (e.g. in artifact markdown without <citations> block).
+ */
+export function syntheticCitationFromLink(href: string, title: string): Citation {
+  return {
+    id: `artifact-cite-${href}`,
+    title: title || href,
+    url: href,
+    snippet: "",
+  };
+}
+
 /**
  * Extract the domain name from a URL for display
  *