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.

client/scripts/feature-pairs.json

@@ -287,6 +287,12 @@
       "desktop": ["src/components/Budget/CostsPanel.tsx"],
       "mobile": ["src/mobile/screens/trip/tabs/MCostsTab.tsx"]
     },
+    {
+      "feature": "Costs/budget splitting or totals only: useCostsMode",
+      "hooks": ["src/components/Budget/useCostsMode.ts"],
+      "desktop": ["src/components/Budget/CostsModeCard.tsx"],
+      "mobile": ["src/mobile/screens/trip/tabs/MCostsTab.tsx"]
+    },
     {
       "feature": "Costs/budget settle-up payment: useSettlementForm",
       "hooks": ["src/components/Budget/useSettlementForm.ts"],

client/scripts/size-baseline.json

@@ -4,7 +4,7 @@
   "src/components/Admin/AdminPluginsPanel.test.tsx": 2704,
   "src/components/Admin/AdminPluginsPanel.tsx": 1266,
   "src/components/Budget/CostsPanel.test.tsx": 2730,
-  "src/components/Budget/CostsPanel.tsx": 1668,
+  "src/components/Budget/CostsPanel.tsx": 1633,
   "src/components/Map/MapView.tsx": 1200,
   "src/components/Map/MapViewGL.test.tsx": 2919,
   "src/components/Map/MapViewGL.tsx": 2126,