refactor: memoize highlight result to avoid redundant tokenization

[TrevorBurnham]

This PR adds useMemo to cache the syntax highlighting result in the CodeView component, preventing expensive re-tokenization on every render.

Problem

The highlight function (which uses ace-code's tokenizer) runs on every render of InternalCodeView, even when content hasn't changed. This is wasteful because:

• Tokenization is O(n) where n is the content length
• It creates many React elements (one span per token, per line)
• Parent re-renders trigger unnecessary re-computation
• Common interactions like hovering action buttons or toggling dark mode cause re-tokenization

Solution

Wrap the highlight call in useMemo with [content, highlight] as dependencies:

const code = useMemo(
  () => (highlight ? highlight(content) : textHighlight(content)),
  [content, highlight]
);

Testing

• All existing unit tests pass
• No changes to component behavior or API

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