feat(costs): let a trip keep its costs as totals only (#1714)

Travellers who track what a trip costs but never split it saw an
Unfinished warning on every expense and a settle-up they had no use for.
A trip now has a costs mode (appended migration, trips.costs_mode, split
by default); totals leaves payers, shares and settling up out of Costs on
desktop and phone through the same gate as a trip of one, and saves new
expenses without a payer. PUT /api/trips/:id and update_trip take it on
budget_edit. The split chips move into their own component.

server/scripts/size-baseline.json

@@ -3,7 +3,7 @@
   "scripts/generate-entities.ts": 2259,
   "src/db/repositories/Places.repository.ts": 1437,
   "src/db/repositories/Reservations.repository.ts": 1559,
-  "src/db/repositories/Trips.repository.ts": 1121,
+  "src/db/repositories/Trips.repository.ts": 1120,
   "src/db/repositories/Users.repository.ts": 1445,
   "src/nest/atlas/atlas.service.ts": 1177,
   "src/nest/auth/auth.service.ts": 1143,