Optimize chat sidebar grouping and message rendering

[leoisadev1]

Summary

• memoize chat grouping/deletion lookups and stabilize day keys to reduce re-renders in the sidebar
• extract the chat message list into a memoized component to streamline rendering and reuse prompt selection helper
• defer model prefetch until after load or idle, skipping on low-bandwidth/save-data connections

Testing

• Not run (not requested)

[github-actions]

🚀 Preview Deployment Ready

Environment	URL
Frontend	https://web-openchat-pr-574.up.railway.app
Convex Dashboard	Dashboard

Convex Preview Backend

• Cloud URL: https://quixotic-chicken-95.convex.cloud
• Site URL: https://quixotic-chicken-95.convex.site

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