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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.28%. Comparing base (a4cabe9) to head (585a37c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
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  Coverage   99.28%   99.28%           
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  Files          29       29           
  Lines         281      281           
  Branches       34       34           
=======================================
  Hits          279      279           
  Misses          1        1           
  Partials        1        1           

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