refactor(frontend): consolidate citation logic, slim exports and impl

- SafeCitationContent: add loadingOnly and renderBody props.
  - loadingOnly: show only loading indicator or null (e.g. write_file step).
  - renderBody(parsed): custom body renderer (e.g. artifact preview).

- message-group write_file: use SafeCitationContent(content, isLoading,
  rehypePlugins, loadingOnly) instead of local useParsedCitations +
  shouldShowCitationLoading + CitationsLoadingIndicator. Pass rehypePlugins
  into ToolCall.

- artifact-file-detail markdown preview: use SafeCitationContent with
  renderBody((p) => <ArtifactFilePreview ... cleanContent={p.cleanContent}
  citationMap={p.citationMap} />). Remove local shouldShowCitationLoading
  and CitationsLoadingIndicator branch.

- core/citations: inline buildCitationMap into use-parsed-citations, remove
  from utils; stop exporting hasCitationsBlock (internal to shouldShowCitationLoading).

- inline-citation: make InlineCitationCard, InlineCitationCardBody,
  InlineCitationSource file-private (no longer exported).

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

---
refactor(前端): 收拢引用逻辑、精简导出与实现

- SafeCitationContent 新增 loadingOnly、renderBody。
  - loadingOnly：仅显示加载或 null（如 write_file 步骤）。
  - renderBody(parsed)：自定义正文渲染（如 artifact 预览）。

- message-group write_file：改用 SafeCitationContent(loadingOnly)，去掉
  本地 useParsedCitations + shouldShowCitationLoading + CitationsLoadingIndicator，
  并向 ToolCall 传入 rehypePlugins。

- artifact-file-detail 的 markdown 预览：改用 SafeCitationContent +
  renderBody 渲染 ArtifactFilePreview，去掉本地加载判断与
  CitationsLoadingIndicator 分支。

- core/citations：buildCitationMap 内联到 use-parsed-citations 并从 utils
  删除；hasCitationsBlock 不再导出（仅 shouldShowCitationLoading 内部使用）。

- inline-citation：InlineCitationCard/Body/Source 改为文件内私有，不再导出。

frontend/src/core/citations/utils.ts

@@ -120,22 +120,6 @@ export function parseCitations(content: string): ParseCitationsResult {
   return { citations, cleanContent };
 }
 
-/**
- * Build a map from URL to Citation for quick lookup
- *
- * @param citations - Array of citations
- * @returns Map with URL as key and Citation as value
- */
-export function buildCitationMap(
-  citations: Citation[],
-): Map<string, Citation> {
-  const map = new Map<string, Citation>();
-  for (const citation of citations) {
-    map.set(citation.url, citation);
-  }
-  return map;
-}
-
 /**
  * Whether the URL is external (http/https).
  */