[Feature]: Compare current tab against on-disk version

[h3pdesign]

Problem

Conflict compare exists only when the app detects a save conflict. There is no general compare-against-disk action when users want to inspect changes on demand.

Proposed solution

Add an explicit command to compare the current tab with its on-disk version without waiting for a conflict state.

Acceptance criteria

• Users can trigger compare-against-disk on demand
• The diff view highlights changed regions clearly
• The command is disabled or explained when no file path exists
• Keyboard navigation works through changed sections
• Accessibility labeling describes the compared sources clearly