Add daily expense tracking with sort/split and CSV export

Standalone expenses (date, description, category, amount) with the same
equal/own/payer split machinery as entry costs, merged into the costs
panel and settle-up as a single expense bucket. Self-fetching card
with per-day grouping, client-side sorting, and quick-add. CSV export
interleaves expenses with priced entries, one share column per member
(UTF-8 BOM, RFC 4180, formula-injection guard on text cells).

Also fixes trip deletion, which hit a foreign-key violation and rolled
back for any trip with checklist items.

public/js/api.js

@@ -94,6 +94,12 @@ export const api = {
     addSuggestions: (tripId, keys, personal) =>
       post(`/api/trips/${tripId}/checklist/suggestions`, personal === undefined ? { keys } : { keys, personal }),
   },
+  expenses: {
+    list: (tripId) => get(`/api/trips/${tripId}/expenses`),
+    create: (tripId, payload) => post(`/api/trips/${tripId}/expenses`, payload),
+    update: (expenseId, patchBody) => patch(`/api/expenses/${expenseId}`, patchBody),
+    remove: (expenseId) => del(`/api/expenses/${expenseId}`),
+  },
 };
 
 export default api;