#8351 feat(chat): add shared find menu

Summary

• Add Cmd/Ctrl-F to chat using the shared find bar and existing highlight colors.
• Navigate matching text across virtualized messages, restore composer focus, and keep streaming from pulling the selected match out of view.
• Reuse displayed mention text and exclude hidden metadata from search.

Type of Change

• New feature

Testing

• Cleanup review across React, state, styling, query, URL state, comments, and test-audit passes.
• 57 focused tests covering chat text parsing, message segmentation, copyable Markdown, and neighboring chat rendering logic.
• App type-check, repository lint, all 51 audits, block-registry check, and docs-manifest check.
• Browser verification with 120 virtualized messages: keyboard navigation and wrapping, Markdown and mention highlights, line breaks, code, no results, capped results, focus restoration, chat switches, and streaming.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)