fix(ui): use oniguruma wasm engine for markdown code highlighting

The JS regex engine in @pierre/diffs can cause catastrophic backtracking
on certain TextMate grammars (e.g. powershell), freezing the main thread.
Switch to a dedicated oniguruma WASM-based highlighter for markdown code
blocks. Diff components still use the existing @pierre/diffs highlighter.
Add try/catch fallback and regression tests.