Add property-based tests for layout computation

[bug-ops]

Parent: #542

Problem

AppLayout::compute is untested with edge-case terminal sizes. Panics or overlapping rects could occur with unusual dimensions.

Requirements

• Add dev-dependency: proptest
• Test AppLayout::compute with arbitrary terminal sizes from 1x1 to 500x500
• Verify: no panics, no overlapping rects, all rects within terminal bounds

Acceptance criteria

• proptest with at least 1000 cases passing
• No panics on any generated size