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.

packages/ui/package.json

@@ -24,6 +24,7 @@
   },
   "scripts": {
     "typecheck": "tsgo --noEmit",
+    "test": "bun test ./src",
     "dev": "vite",
     "generate:tailwind": "bun run script/tailwind.ts"
   },