refactor: split currency and key/metal formatting into separate units

this simplifies functions and tests, and will make currency optional/variable later

__tests__/currency.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, test, jest, mock, beforeEach } from "bun:test";
+import { convertTF2PriceToUSD } from '../src/content/utils/currency'
+
+mock.module('../src/content/config', () => ({
+  conversion_ref_usd: 0.05 // Mock conversion rate
+}))
+
+
+describe('Currency Service', () => {
+  beforeEach(() => {
+    jest.clearAllMocks()
+  });
+
+  test('convertTF2PriceToUSD returns correct price', () => {
+    expect(convertTF2PriceToUSD(1, 10, 50)).toBe(3);
+  })
+})