Vacay drag-to-paint, "Everyone" button, live exchange rates

- Vacay: click-and-drag to paint/erase vacation days across calendar
- Vacay: "Everyone" button sets days for all persons (2+ only)
- Budget: live currency conversion via frankfurter.app (cached 1h)
- Budget: conversion widget in total card with selectable target currency
- Day planner: remove transport mode buttons from day view

client/src/api/client.js

@@ -166,6 +166,10 @@ export const reservationsApi = {
   delete: (tripId, id) => apiClient.delete(`/trips/${tripId}/reservations/${id}`).then(r => r.data),
 }
 
+export const exchangeApi = {
+  getRates: () => apiClient.get('/exchange-rates').then(r => r.data),
+}
+
 export const weatherApi = {
   get: (lat, lng, date) => apiClient.get('/weather', { params: { lat, lng, date, units: 'metric' } }).then(r => r.data),
 }