Design: acting on a limb uses the design system's row and dialog, and limb-* leave §5d

Part of #2678. src/web/pages/limb.ts has its own .limb-confirm, .limb-rows and .limb-why.

Done:

[ ] The confirm step is the §5 dialog and row, and the three classes and their rules are gone.
[ ] Its test still proves the action needs the confirm and a same-origin POST. Done also means the pull's looks show the page before and after, desktop and 360px, light and dark. Every class it stops using leaves docs/DESIGN.md §5d in the same pull, and bun scripts/design-lint.ts passes.

Files: src/web/pages/limb.ts, docs/DESIGN.md, its test.

Design: acting on a limb uses the design system's row and dialog, and limb-* leave §5d #3185