refactor(android): add shared formatDuration + ms/sec conversion helpers

android/app/src/main/java/com/fabledsword/minstrel/shared/Formatting.kt

@@ -0,0 +1,23 @@
+package com.fabledsword.minstrel.shared
+
+const val MILLIS_PER_SECOND = 1000
+
+private const val SECONDS_PER_MINUTE = 60
+
+/**
+ * Formats [seconds] as m:ss. Returns [zero] when [seconds] <= 0 — pass
+ * "--:--" for unknown track durations, "0:00" (default) for playback
+ * position/duration. The single mm:ss formatter for the client.
+ */
+fun formatDuration(seconds: Int, zero: String = "0:00"): String =
+    if (seconds <= 0) {
+        zero
+    } else {
+        "%d:%02d".format(seconds / SECONDS_PER_MINUTE, seconds % SECONDS_PER_MINUTE)
+    }
+
+fun Int.secondsToMillis(): Int = this * MILLIS_PER_SECOND
+
+fun Int.millisToSeconds(): Int = this / MILLIS_PER_SECOND
+
+fun Long.millisToSeconds(): Int = (this / MILLIS_PER_SECOND).toInt()

android/app/src/test/java/com/fabledsword/minstrel/shared/FormattingTest.kt

@@ -0,0 +1,27 @@
+package com.fabledsword.minstrel.shared
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class FormattingTest {
+    @Test
+    fun `formatDuration formats minutes and seconds`() {
+        assertEquals("0:05", formatDuration(5))
+        assertEquals("2:05", formatDuration(125))
+        assertEquals("10:00", formatDuration(600))
+    }
+
+    @Test
+    fun `formatDuration uses the zero string for non-positive input`() {
+        assertEquals("0:00", formatDuration(0))
+        assertEquals("0:00", formatDuration(-3))
+        assertEquals("--:--", formatDuration(0, zero = "--:--"))
+    }
+
+    @Test
+    fun `conversions round-trip seconds and millis`() {
+        assertEquals(5000, 5.secondsToMillis())
+        assertEquals(5, 5000.millisToSeconds())
+        assertEquals(5, 5000L.millisToSeconds())
+    }
+}