Edit slots from the review page

Needs your OK: this adds slot editing to the review page (#502). Swap pipeline:proposed for pipeline:ready to have it built. Part of #498.

What

Tap a slot to edit it. The builder's existing SlotEditor opens under the slot and saves through the same PATCH /api/slots/[id]. There's one editor and one save path, not a second editor to maintain.
"People for each slot" sets every slot's capacity at once. It uses the existing slots/bulk route, which no UI calls today.
"Remove day" on each date group header.
Nothing else. No new layouts; anything bigger goes to the builder through "Edit everything in the builder".

Design: canvas https://claude.ai/artifact/RqcjzCcdSTw5JhbXjm2x5y, row C, artboard "C · After the two fixes, editing one slot". The canvas is private until shared.

Saving. The builder saves 800 ms after typing stops, with no flush when the page unloads. In the card, use an explicit Done so a quick close can't lose an edit.
Undo for Remove day. On a draft it deletes slots outright, so offer Undo, as the fixes in Find fixable problems in a draft and offer one-tap fixes #503 do.
Someone else editing. An AI assistant can change the same draft at the same time (see After a settings save, the Build tab can point reminders at a field it hasn't loaded #398). Refresh after saves.
JS budget. The editor is a client island. Keep the page's entry in budgets.json honest.

A slot's values and capacity can be changed from the review page and are saved through the existing route.
"People for each slot" and "Remove day" work on a draft and can be undone.
pnpm lint && pnpm typecheck && pnpm knip && pnpm test pass, and pnpm budgets passes.