Adds API endpoint for default currency

Adds an API endpoint to retrieve the default accounting
currency configured for the LNbits instance. Returns the configured
default currency or None if not set.

lnbits/core/views/api.py

@@ -100,6 +100,16 @@ async def api_list_currencies_available() -> list[str]:
     return allowed_currencies()
 
 
+@api_router.get("/api/v1/default-currency")
+async def api_get_default_currency() -> dict[str, str | None]:
+    """
+    Get the default accounting currency for this LNbits instance.
+    Returns the configured default, or None if not set.
+    """
+    default_currency = settings.lnbits_default_accounting_currency
+    return {"default_currency": default_currency}
+
+
 @api_router.post("/api/v1/conversion")
 async def api_fiat_as_sats(data: ConversionData):
     output = {}