perf(webview): memoize MessageItem/MessageList and remove click-to-focus

Extracted from #39. Wrap MessageItem and MessageList in React.memo to cut streaming-time re-renders, move the focus ring into UserMessage, and add a subtle hover ring.

Deliberate behavior removal to state in the PR: clicking a message no longer syncs the keyboard-nav index (focus becomes keyboard-only via Cmd/Ctrl+Up/Down).

**Size**: S | **Depends on**: nothing; complements #70 and can fold into it | **Tier**: 2

Look for MessageItem and MessageList components in the webview code. Apply React.memo to them. Move the focus ring logic into UserMessage and add a hover style. Verify the click-to-focus behavior is removed as described. Test that streaming re-renders are reduced and keyboard navigation still works with Cmd/Ctrl+Up/Down.

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