Make GPU work and regions divider resizable

Use the existing egui panel resizing behavior for the vertical split between GPU work and GPU regions. Keep independent scrolling, reserve usable space for both sections, and retain the split while navigating frames/pass details. Verified with synthetic egui pointer dragging: resizing in both directions, bounds, visible regions, and retained height across frame/pass navigation. cargo check --workspace, cargo test --workspace (50 tests), cargo fmt --all -- --check, and git diff --check pass. No live Wayland visual check performed.