merge: upstream/experimental with citations feature

- Merge upstream changes including image search, tooltips, and UI improvements
- Keep citations feature with inline hover cards
- Resolve conflict in message-list-item.tsx: use upstream img max-width (90%) while preserving citations logic
- Maintain file upload improvements with citations support

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

backend/src/agents/lead_agent/prompt.py

@@ -158,6 +158,7 @@ The key AI trends for 2026 include enhanced reasoning capabilities, multimodal i
 - Progressive Loading: Load resources incrementally as referenced in skills
 - Output Files: Final deliverables must be in `/mnt/user-data/outputs`
 - Clarity: Be direct and helpful, avoid unnecessary meta-commentary
+- Including Images and Mermaid: Images and Mermaid diagrams are always welcomed in the Markdown format, and you're encouraged to use `![Image Description](image_path)\n\n` or "```mermaid" to display images in response or Markdown files
 - Multi-task: Better utilize parallel tool calling to call multiple tools at one time for better performance
 - Language Consistency: Keep using the same language as user's
 - Always Respond: Your thinking is internal. You MUST always provide a visible response to the user after thinking.