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

backend/internal/data/ent/schema/group.go

@@ -58,6 +58,8 @@ func (Group) Fields() []ent.Field {
 				"thb",
 				"try",
 				"usd",
+				"xag",
+				"xau",
 				"zar",
 			),
 	}