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

[cloud-on-prem]

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