feat: add currencies XAG and XAU (#535)

* Added currencies XAG and XAU to currency.ts

I added XAG and XAU for myself and others who prefer to measure value with something of substance.

Review the ISO 4217 standard to view a full list of official currency codes including the ones I have added.

https://www.iso.org/iso-4217-currency-codes.html
https://en.wikipedia.org/wiki/ISO_4217

Example:
https://www.xe.com/currencyconverter/convert/?Amount=100&From=XAG&To=USD

API for exchange rates:
https://openexchangerates.org/

* Added field values xag and xau to group.go

* Update group.go
Former-commit-id: 5438898b49

frontend/lib/data/currency.ts

@@ -27,6 +27,8 @@ export type Codes =
   | "THB"
   | "TRY"
   | "USD"
+  | "XAG"
+  | "XAU"
   | "ZAR";
 
 export type Currency = {
@@ -65,5 +67,7 @@ export const currencies: Currency[] = [
   { code: "THB", local: "Thailand", symbol: "฿", name: "Thai Baht" },
   { code: "TRY", local: "Turkey", symbol: "₺", name: "Turkish Lira" },
   { code: "USD", local: "United States", symbol: "$", name: "United States Dollar" },
+  { code: "XAG", local: "Global", symbol: "XAG", name: "Silver Troy Ounce" },
+  { code: "XAU", local: "Global", symbol: "XAU", name: "Gold Troy Ounce" },
   { code: "ZAR", local: "South Africa", symbol: "R", name: "South African Rand" },
 ];