feat(citations): add shared citation components and optimize code

## New Features
- Add `CitationLink` shared component for rendering citation hover cards
- Add `CitationsLoadingIndicator` component for showing loading state
- Add `removeAllCitations` utility to strip all citations from content
- Add backend support for removing citations when downloading markdown files
- Add i18n support for citation loading messages (en-US, zh-CN)

## Code Optimizations
- Remove duplicate `ExternalLinkBadge` component, reuse `CitationLink` instead
- Consolidate `remarkPlugins` config in `streamdownPlugins` to avoid duplication
- Remove unused imports: `Citation`, `buildCitationMap`, `extractDomainFromUrl`, etc.
- Remove unused `messages` parameter from `ToolCall` component
- Remove unused `isWriteFile` parameter from `ArtifactFilePreview` component
- Remove unused `useI18n` hook from `MessageContent` component

## Bug Fixes
- Fix `remarkGfm` plugin configuration that prevented table rendering
- Fix React Hooks rule violation: move `useMemo` to component top level
- Replace `||` with `??` for nullish coalescing in clipboard data

## Code Cleanup
- Remove debug console.log/info statements from:
  - `threads/hooks.ts`
  - `notification/hooks.ts`
  - `memory-settings-page.tsx`
- Fix import order in `message-group.tsx`

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

frontend/src/components/workspace/settings/memory-settings-page.tsx

@@ -37,8 +37,6 @@ function memoryToMarkdown(
 ) {
   const parts: string[] = [];
 
-  console.info(memory);
-
   parts.push(`## ${t.settings.memory.markdown.overview}`);
   parts.push(`- **${t.common.version}**: \`${memory.version}\``);
   parts.push(