Add keyboard navigation to Thread message list

[jkbrooks]

Feature Request

Add comprehensive keyboard navigation support to the Thread component's message list, enabling users to navigate through messages without a mouse.

Background

The packages/react/src/primitives/thread/ components currently render a scrollable list of messages via ThreadMessages.tsx and ThreadViewport.tsx. This feature adds accessibility-focused keyboard navigation.

Requirements

1. Navigation Keys

• Arrow Up/Down: Move focus to previous/next message
• Home: Jump to first message
• End: Jump to last message
• Page Up/Page Down: Move focus by ~5 messages (or viewport height)

2. Focus Management

• Add tabIndex and focus handling to MessageRoot.tsx
• Visual focus indicator (ring/outline) when message is focused
• Focus should follow scroll (auto-scroll to focused message)
• Initial focus should be on the most recent message when entering the list

3. Accessibility (ARIA)

• role="listbox" on the message container
• role="option" on individual messages
• aria-activedescendant for current focus
• aria-label for screen reader context

4. Integration Points

Files likely to be modified:

• packages/react/src/primitives/thread/ThreadMessages.tsx
• packages/react/src/primitives/thread/ThreadViewport.tsx
• packages/react/src/primitives/message/MessageRoot.tsx
• Possibly create a new hook: useThreadKeyboardNavigation.tsx

Acceptance Criteria

• Can navigate entire message list using only keyboard
• Focus is clearly visible on the active message
• Screen readers announce navigation (test with VoiceOver/NVDA)
• No breaking changes to existing mouse/touch interactions
• Keyboard navigation works in all Thread variants
• Unit tests for keyboard event handlers
• Navigation state resets appropriately on new messages

Non-Goals

• This PR does NOT implement:
  • Message content editing via keyboard
  • Keyboard shortcuts for actions (copy, delete, etc.)
  • Vim-style navigation (j/k)

Example Usage

// Should work automatically when using Thread.Viewport
<Thread.Root>
  <Thread.Viewport>
    <Thread.Messages />  {/* Now keyboard navigable */}
  </Thread.Viewport>
  <Composer />
</Thread.Root>

Testing Notes

Test keyboard navigation:

1. Tab into the message list
2. Press Down Arrow - should focus first/next message
3. Press Up Arrow - should focus previous message
4. Press Home - should focus first message
5. Press End - should focus last message
6. Verify screen reader announces "Message X of Y"