Virtualize chat timeline rendering

[andreivince]

Summary

• Add a centralized virtual timeline window helper for large loaded chat windows
• Render only the active message row window in ConversationView while keeping the existing message bubble/media path
• Strengthen the large-chat visual test to prove DOM rows stay bounded after expanding earlier messages

Verification

• npm test
• npm run build
• npm run hygiene:public
• npm run release:readiness
• npm run visual:check
• Browser QA on http://127.0.0.1:1420/?demo=large-chat: fresh load showed latest message with 36 rows mounted; expanded state showed 840 loaded messages with 45 rows mounted

Note: cargo test --workspace was started but interrupted after several minutes of no output and no active child process; this PR does not touch Rust.

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