feat: add analytics event tracking for leaderboard interactions

[q275343119]

Summary

This PR adds anonymous analytics event tracking to the leaderboard frontend, so user behavior can be collected by the new analytics pipeline and visualized in the analytics dashboard.

Related work:

• Analytics collector service: https://github.com/embedding-benchmark/analytics-collector
• Analytics dashboard frontend: https://github.com/embedding-benchmark/leaderboard-analytics-frontend

Changes

• Added a typed analytics client that batches events and posts them to PUBLIC_ANALYTICS_URL.
• Added anonymous visitor/session IDs using localStorage and sessionStorage.
• Added analytics env flags:
  • PUBLIC_ANALYTICS_URL
  • PUBLIC_ANALYTICS_ENABLED
• Tracked key leaderboard interactions:
  • page views
  • search changes
  • filter changes
  • sort changes
  • tab selection
  • model pin/unpin
  • compare page open / model changes
  • CSV downloads
  • share link copy
  • external link clicks
• Added an analytics collector contract doc covering endpoint shape, event schema, privacy constraints, and suggested storage/indexing.
• Avoided sending raw query values or raw search text; only query keys and search length are sent.

Privacy Notes

• No raw search query text is sent.
• URL query values are not sent; only query parameter names are included.
• IDs are anonymous client-generated visitor/session identifiers.

Preview

• Leaderboard frontend: https://huggingface.co/spaces/SmileXing/leaderboard-frontend
• Analytics collector service: https://huggingface.co/spaces/SmileXing/analytics-collector
• Analytics dashboard frontend: https://huggingface.co/spaces/SmileXing/leaderboard-analytics-frontend