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";