feat(diff-viewer): add diff viewer utility with unified diff output

[berkdurmus]

Summary

Screen.Recording.2026-01-24.at.18.37.06.mov

• Add new Diff Viewer utility with Monaco DiffEditor supporting text/code, JSON, YAML, and config diffs.
• Normalize structured inputs before diffing and expose inline/side-by-side, language, and ignore-whitespace controls.
• Include unified diff output, diff stats, and register the tool in the utilities list.

Test Plan

• Open /utilities/diff-viewer and paste sample inputs for each mode.
• Toggle inline/side-by-side and ignore-whitespace; verify diff updates.
• Use the language selector for code mode; confirm highlighting.
• Click “Copy unified diff” and paste to confirm output.