Channel message selection

[peterchinman]

Restore arrow key navigation to select adjacent messages in channels instead of scrolling the message list.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[github-actions]

Preview: https://cursor-channel-message-selecti-2gjdb5.preview.macro.com/app (6251edf)

[claude]

Code review

This PR introduces significant navigation behavior changes but does not include any tests, which violates the testing requirement in CLAUDE.md.

Issue: Missing Tests

Rule violated: CLAUDE.md Rule 4 states:

"Write tests for your changes. When fixing bugs or regressions, identify the issue with a test before fixing it."

Reference: js/app/CLAUDE.md

Context: This PR modifies core navigation logic across multiple files (~156 additions, ~104 deletions), including:

• New navigation functions (navigateChannel, navigateToMessage, navigateByOffset, etc.)
• Complex hybrid navigation logic combining DOM-based and ID-based approaches
• Changes to focus management and keyboard event handling

Recommendation: Add tests to verify:

• Arrow key navigation selects adjacent messages correctly
• Hybrid navigation fallback works when DOM elements are unmounted during virtualization
• Mouse hover updates selected message appropriately
• Focus/blur behavior maintains correct selected message state

The navigation logic contains testable business logic that could be decoupled from the UI layer and tested using bun run test. For UI-specific interactions, consider using Playwright.