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/components/ai-elements/inline-citation.tsx

@@ -29,35 +29,31 @@ import {
 import { Shimmer } from "./shimmer";
 import { useI18n } from "@/core/i18n/hooks";
 
-export type InlineCitationCardProps = ComponentProps<typeof HoverCard>;
+type InlineCitationCardProps = ComponentProps<typeof HoverCard>;
 
-export const InlineCitationCard = (props: InlineCitationCardProps) => (
+const InlineCitationCard = (props: InlineCitationCardProps) => (
   <HoverCard closeDelay={0} openDelay={0} {...props} />
 );
 
-export type InlineCitationCardBodyProps = ComponentProps<"div">;
-
-export const InlineCitationCardBody = ({
+const InlineCitationCardBody = ({
   className,
   ...props
-}: InlineCitationCardBodyProps) => (
+}: ComponentProps<"div">) => (
   <HoverCardContent className={cn("relative w-80 p-0", className)} {...props} />
 );
 
-export type InlineCitationSourceProps = ComponentProps<"div"> & {
-  title?: string;
-  url?: string;
-  description?: string;
-};
-
-export const InlineCitationSource = ({
+const InlineCitationSource = ({
   title,
   url,
   description,
   className,
   children,
   ...props
-}: InlineCitationSourceProps) => (
+}: ComponentProps<"div"> & {
+  title?: string;
+  url?: string;
+  description?: string;
+}) => (
   <div className={cn("space-y-1", className)} {...props}>
     {title && (
       <h4 className="truncate font-medium text-sm leading-tight">{title}</h4>