Rate history chart and pension contribution calculator

Analytics gains a 90-day USD/CRC buy/sell chart from the existing
history endpoint (wishlist: rate history). Pensiones gains a
Calculadora de Aporte: fund + target amount + target age → required
monthly contribution via the annuity future-value formula, using the
fund's live balance and configured rate (wishlist: contribution
calculator).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -337,3 +337,10 @@ export type UserSettingsData = Record<string, unknown>;
 export type UserSettingsRead = Schema<'UserSettingsRead'>;
 
 export const getUserSettings = () => api.get<UserSettingsRead>('/settings/');
+
+// --- Exchange Rate ---
+
+export type ExchangeRatePoint = Schema<'ExchangeRateRead'>;
+
+export const getRateHistory = (days = 90) =>
+  api.get<ExchangeRatePoint[]>('/exchange-rate/history', { params: { days } });