fix: show 0 balance with decimals in classic denomination

app/src/main/java/to/bitkit/models/Currency.kt

@@ -106,12 +106,16 @@ fun Long.formatToModernDisplay(locale: Locale = Locale.getDefault()): String {
 fun ULong.formatToModernDisplay(locale: Locale = Locale.getDefault()): String = toLong().formatToModernDisplay(locale)
 
 fun Long.formatToClassicDisplay(locale: Locale = Locale.getDefault()): String {
-    val sats = this
     val symbols = DecimalFormatSymbols(locale).apply {
         decimalSeparator = DECIMAL_SEPARATOR
     }
-    val formatter = DecimalFormat("###.########", symbols)
-    return formatter.format(sats.asBtc())
+    val pattern = "0.${"0".repeat(CLASSIC_DECIMALS)}"
+    val formatter = DecimalFormat(pattern, symbols).apply {
+        minimumFractionDigits = CLASSIC_DECIMALS
+        maximumFractionDigits = CLASSIC_DECIMALS
+        isGroupingUsed = false
+    }
+    return formatter.format(asBtc())
 }
 
 fun BigDecimal.formatCurrency(decimalPlaces: Int = FIAT_DECIMALS, locale: Locale = Locale.getDefault()): String? {

app/src/test/java/to/bitkit/models/CurrencyTest.kt

@@ -0,0 +1,40 @@
+package to.bitkit.models
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.util.Locale
+
+class CurrencyTest {
+
+    @Test
+    fun `formatToModernDisplay uses space grouping`() {
+        val sats = 123_456_789L
+
+        val formatted = sats.formatToModernDisplay(Locale.US)
+
+        assertEquals("123 456 789", formatted)
+    }
+
+    @Test
+    fun `formatToModernDisplay handles zero`() {
+        val formatted = 0L.formatToModernDisplay(Locale.US)
+
+        assertEquals("0", formatted)
+    }
+
+    @Test
+    fun `formatToClassicDisplay always shows eight decimals`() {
+        val formatted = 0L.formatToClassicDisplay(Locale.US)
+
+        assertEquals("0.00000000", formatted)
+    }
+
+    @Test
+    fun `formatToClassicDisplay converts sats to btc`() {
+        val sats = 12_345L // 0.00012345 BTC
+
+        val formatted = sats.formatToClassicDisplay(Locale.US)
+
+        assertEquals("0.00012345", formatted)
+    }
+}