Add response timestamp to API and display in chat UI

[mateenasadaf]

Overview

As part of getting familiar with the VCell-AI codebase, I made two small
improvements — one on the backend and one on the frontend.

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Changes

Backend — backend/app/routes/llms_router.py

• Added a timestamp field to the /query endpoint response
• The timestamp is in UTC ISO-8601 format (e.g. 2025-03-24T10:45:00+00:00)
• Used datetime.now(timezone.utc) instead of datetime.utcnow() to avoid
  naive datetime bugs when the server and client are in different timezones
• Added thorough docstrings to all four endpoints explaining their purpose,
  arguments, and return values

Frontend — frontend/components/ChatBox.tsx

• Added a small time label beneath each message bubble (e.g. "02:34 PM")
• User messages show the time in text-blue-200 (right-aligned)
• Assistant messages show the time in text-slate-400 (left-aligned)
• When the backend returns a timestamp, it is used directly so the displayed
  time reflects when the server generated the response, not when the client
  received it — with a graceful fallback to new Date() for compatibility
• Added inline comments throughout explaining all major logic blocks

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Why these changes?

• The timestamp makes it easy to see when each response was produced,
  which helps with debugging and gives users a better sense of response times
• The comments make the codebase easier to onboard into for new contributors

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Testing

• No existing functionality was changed — both changes are purely additive
• The frontend falls back gracefully if the backend does not return a timestamp