[Feature]: Compare two open tabs in a native diff view

[h3pdesign]

Problem

The app can compare local and remote conflict states, but it cannot compare two arbitrary open tabs as a first-class workflow.

Proposed solution

Add a native compare flow for any two open tabs, reusing future diff infrastructure instead of separate one-off compare sheets.

Acceptance criteria

• Users can choose two open tabs to compare
• The diff surface makes insertions, deletions, and changed blocks clear
• Navigation between change regions is available
• The feature works on macOS, iPhone, and iPad within platform-appropriate layouts
• VoiceOver and keyboard users can move through the comparison reliably