Add commit diff fetching and display enhancements

[beNative]

Summary

• add an IPC handler to retrieve commit diffs for Git and SVN repositories and expose it through the preload layer
  to the renderer
• extend the commit history modal to request, cache, filter, and render commit diffs with copy and collapse controls
• style the diff presentation to handle large changes with pagination and syntax highlighting cues

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_68e11c5604808332ac79caf54924b924