Match the design's `0 1px 0 rgba(0,0,0,0.05), 0 12px 30px -16px
rgba(0,0,0,0.18)` shadow stack on every split pane card and tighten
inter-pane gap from 12 px (`gap_3`) to the design's 10 px. Without the
ambient shadow the panes read as flat seams against the wallpaper;
with it they sit on the canvas the way the design renders them.
Grid axis now also gets a 10 px row gap so the second row doesn't
butt up against the first.
cargo test --workspace: 440 passed, 0 failed.
ZacharyZhang-NY
c85023170c