Add option to collapse unchanged lines in desktop diff viewer

Description

In the Claude Code desktop app, the diff viewer shows the full file content with changes highlighted. There's no way to collapse the unchanged regions and view only the changed hunks (with optional surrounding context), the way git diff or GitHub PR diffs do by default.

Problem

For large files, the unchanged content dominates the view and you have to scroll to find the actual changes. This makes reviewing changes slower than it needs to be.

Proposed solution

Add a toggle (and/or a setting) to the diff viewer that collapses unchanged lines, showing only the changed hunks plus a few lines of context — similar to GitHub's diff view, with an option to expand collapsed sections on demand.

Environment

• Claude Code desktop app (Mac)