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/lib/utils.ts

@@ -1,6 +1,12 @@
-import { clsx, type ClassValue } from "clsx"
-import { twMerge } from "tailwind-merge"
+import { clsx, type ClassValue } from "clsx";
+import { twMerge } from "tailwind-merge";
 
 export function cn(...inputs: ClassValue[]) {
-  return twMerge(clsx(inputs))
+  return twMerge(clsx(inputs));
 }
+
+/** Shared class for external links (underline by default). */
+export const externalLinkClass =
+  "text-primary underline underline-offset-2 hover:no-underline";
+/** For streaming / loading state when link may be a citation (no underline). */
+export const externalLinkClassNoUnderline = "text-primary hover:underline";