Add displayDecimals to the currency object.

This helps the sender display the receiving currency amount correctly in their UI.

Protocol change: uma-universal-money-address/protocol#7

packages/core/src/Currency.ts

@@ -1,12 +1,46 @@
 import { z } from "zod";
 
 export const CurrencySchema = z.object({
+  /**
+   * The currency code, eg. "USD".
+   */
   code: z.string(),
+
+  /**
+   * The full currency name, eg. "US Dollars".
+   */
   name: z.string(),
+
+  /**
+   * The symbol of the currency, eg. "$".
+   */
   symbol: z.string(),
-  multiplier: z.number(), // millisatoshi per unit
+
+  /**
+   * Estimated millisats per smallest "unit" of this currency (eg. 1 cent in USD).
+   */
+  multiplier: z.number(),
+
+  /**
+   * Minimum amount that can be sent in this currency. This is in the smallest unit of the currency
+   * (eg. cents for USD).
+   */
   minSendable: z.number(),
+
+  /**
+   * Maximum amount that can be sent in this currency. This is in the smallest unit of the currency
+   * (eg. cents for USD).
+   */
   maxSendable: z.number(),
+
+  /**
+   * Number of digits after the decimal point for display on the sender side. For example,
+   * in USD, by convention, there are 2 digits for cents - $5.95. in this case, `displayDecimals`
+   * would be 2. Note that the multiplier is still always in the smallest unit (cents). This field
+   * is only for display purposes. The sender should assume zero if this field is omitted, unless
+   * they know the proper display format of the target currency.
+   */
+  displayDecimals: z.optional(z.number()),
 });
 
 export type Currency = z.infer<typeof CurrencySchema>;

packages/core/src/tests/uma.test.ts

@@ -282,6 +282,7 @@ describe("uma", () => {
           multiplier: 34_150,
           minSendable: 1,
           maxSendable: 10_000_000,
+          displayDecimals: 2,
         },
       ],
       receiverKycStatus: KycStatus.Verified,