fix: Use legacy diff algorithm for better large file performance

- Set diffAlgorithm to 'legacy' for Monaco DiffEditor
- Add maxComputationTime of 10s to avoid UI freeze on huge files

This addresses the issue where sessions with large JSON files (50k-100k+ lines)
would cause the UI to freeze. The 'legacy' algorithm is faster than 'advanced'
for large files, similar to VSCode's workaround for the same issue.

See: https://github.com/microsoft/vscode/issues/184037

packages/ui/src/components/file-viewer/monaco-diff-viewer.tsx

@@ -61,6 +61,11 @@ export function MonacoDiffViewer(props: MonacoDiffViewerProps) {
         // Keep enough gutter space so unified diffs don't overlap `+`/`-` markers.
         lineNumbersMinChars: 4,
         lineDecorationsWidth: 12,
+        // Use legacy diff algorithm for better performance with large files
+        // See: https://github.com/microsoft/vscode/issues/184037
+        diffAlgorithm: "legacy",
+        // Limit computation time to avoid freezing on large files
+        maxComputationTime: 10000,
       })
 
       setReady(true)