feat: Add currency support to billing data and enhance BillingManager with exchange rate fetching and formatting functions

example-servers-billing.json

@@ -7,9 +7,11 @@
     "provider": "VDSINA",
     "loginUrl": "cp.vdsina.com",
     "monthlyCost": 12.00,
+    "monthlyCostCurrency": "USD",
     "nextPaymentDate": "2026-04-25",
     "lastPaymentDate": "2025-05-05",
     "lastPaymentAmount": 12.00,
+    "lastPaymentCurrency": "USD",
     "status": "active",
     "notes": ""
   },
@@ -21,9 +23,11 @@
     "provider": "Macloud",
     "loginUrl": "cp.macloud.ru",
     "monthlyCost": 7.00,
+    "monthlyCostCurrency": "USD",
     "nextPaymentDate": "2025-09-02",
     "lastPaymentDate": "2025-02-03",
     "lastPaymentAmount": 7.00,
+    "lastPaymentCurrency": "USD",
     "status": "active",
     "notes": ""
   },
@@ -35,9 +39,11 @@
     "provider": "Hosting VDS",
     "loginUrl": "my.hosting-vds.com/",
     "monthlyCost": 11.95,
+    "monthlyCostCurrency": "USD",
     "nextPaymentDate": "2025-12-21",
     "lastPaymentDate": "2025-06-13",
     "lastPaymentAmount": 10.00,
+    "lastPaymentCurrency": "USD",
     "status": "active",
     "notes": ""
   },
@@ -49,9 +55,11 @@
     "provider": "Waicore",
     "loginUrl": "my.waicore.com/billmgr",
     "monthlyCost": 9.60,
+    "monthlyCostCurrency": "USD",
     "nextPaymentDate": "2026-04-25",
     "lastPaymentDate": "2025-04-25",
     "lastPaymentAmount": 9.60,
+    "lastPaymentCurrency": "USD",
     "status": "active",
     "notes": ""
   },
@@ -63,9 +71,11 @@
     "provider": "IHOR",
     "loginUrl": "billing.ihor-hosting.ru/billmgr",
     "monthlyCost": 10.00,
+    "monthlyCostCurrency": "USD",
     "nextPaymentDate": "2025-09-10",
     "lastPaymentDate": "2025-06-25",
     "lastPaymentAmount": 10.00,
+    "lastPaymentCurrency": "USD",
     "status": "active",
     "notes": ""
   }