refactor(util): extract FormatUtils and deduplicate byte formatting

Consolidate formatBytes and formatDuration into util/FormatUtils.
Use in SystemScreen and MemoryScreen, backed by unit tests.

app/src/main/java/com/m57/hermescontrol/ui/memory/MemoryScreen.kt

@@ -62,6 +62,7 @@ import com.m57.hermescontrol.ui.common.listContentPadding
 import com.m57.hermescontrol.ui.common.listItemSpacing
 import com.m57.hermescontrol.ui.plugins.memoryProviderStatusLabel
 import com.m57.hermescontrol.ui.plugins.memoryProviderStatusType
+import com.m57.hermescontrol.util.formatBytes
 
 /**
  * Memory management home — owns the memory surface that used to live in the
@@ -425,10 +426,3 @@ private fun MemoryProviderRow(
         }
     }
 }
-
-private fun formatBytes(bytes: Long): String =
-    when {
-        bytes >= 1024 * 1024 -> "%.1f MB".format(bytes / (1024.0 * 1024.0))
-        bytes >= 1024 -> "%.1f KB".format(bytes / 1024.0)
-        else -> "$bytes B"
-    }

app/src/main/java/com/m57/hermescontrol/ui/system/SystemScreen.kt

@@ -96,31 +96,12 @@ import com.m57.hermescontrol.ui.common.StatusBadgeType
 import com.m57.hermescontrol.ui.common.ToastEffect
 import com.m57.hermescontrol.ui.system.components.CredentialEntryRow
 import com.m57.hermescontrol.ui.system.components.HookCard
+import com.m57.hermescontrol.util.formatBytes
+import com.m57.hermescontrol.util.formatDuration
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 
-// ── Helpers ─────────────────────────────────────────────────────────────
-
-private fun formatBytes(bytes: Long): String =
-    when {
-        bytes < 1024 -> "$bytes B"
-        bytes < 1024 * 1024 -> "${bytes / 1024} KB"
-        bytes < 1024 * 1024 * 1024 -> "${bytes / (1024 * 1024)} MB"
-        else -> "${"%.1f".format(bytes.toDouble() / (1024 * 1024 * 1024))} GB"
-    }
-
-private fun formatDuration(totalSeconds: Double): String {
-    val days = (totalSeconds / 86400).toInt()
-    val hours = ((totalSeconds % 86400) / 3600).toInt()
-    val minutes = ((totalSeconds % 3600) / 60).toInt()
-    return buildString {
-        if (days > 0) append("${days}d ")
-        if (hours > 0 || days > 0) append("${hours}h ")
-        append("${minutes}m")
-    }
-}
-
 @OptIn(androidx.compose.foundation.layout.ExperimentalLayoutApi::class)
 @Composable
 fun SystemScreen(

app/src/main/java/com/m57/hermescontrol/util/FormatUtils.kt

@@ -0,0 +1,28 @@
+package com.m57.hermescontrol.util
+
+import java.util.Locale
+
+/**
+ * Formats a byte count into human-readable B, KB, MB, or GB.
+ */
+fun formatBytes(bytes: Long): String =
+    when {
+        bytes < 1024 -> "$bytes B"
+        bytes < 1024 * 1024 -> "${bytes / 1024} KB"
+        bytes < 1024 * 1024 * 1024 -> "${bytes / (1024 * 1024)} MB"
+        else -> String.format(Locale.US, "%.1f GB", bytes.toDouble() / (1024 * 1024 * 1024))
+    }
+
+/**
+ * Formats seconds into a human-readable duration (e.g. "2d 4h 15m").
+ */
+fun formatDuration(totalSeconds: Double): String {
+    val days = (totalSeconds / 86400).toInt()
+    val hours = ((totalSeconds % 86400) / 3600).toInt()
+    val minutes = ((totalSeconds % 3600) / 60).toInt()
+    return buildString {
+        if (days > 0) append("${days}d ")
+        if (hours > 0 || days > 0) append("${hours}h ")
+        append("${minutes}m")
+    }
+}

app/src/test/java/com/m57/hermescontrol/util/FormatUtilsTest.kt

@@ -0,0 +1,54 @@
+package com.m57.hermescontrol.util
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class FormatUtilsTest {
+    @Test
+    fun testFormatBytes_bytes() {
+        assertEquals("0 B", formatBytes(0L))
+        assertEquals("500 B", formatBytes(500L))
+        assertEquals("1023 B", formatBytes(1023L))
+    }
+
+    @Test
+    fun testFormatBytes_kilobytes() {
+        assertEquals("1 KB", formatBytes(1024L))
+        assertEquals("2 KB", formatBytes(2048L))
+        assertEquals("1023 KB", formatBytes(1024 * 1024 - 1L))
+    }
+
+    @Test
+    fun testFormatBytes_megabytes() {
+        assertEquals("1 MB", formatBytes(1024 * 1024L))
+        assertEquals("500 MB", formatBytes(500 * 1024 * 1024L))
+    }
+
+    @Test
+    fun testFormatBytes_gigabytes() {
+        assertEquals("1.0 GB", formatBytes(1024 * 1024 * 1024L))
+        assertEquals("2.5 GB", formatBytes((2.5 * 1024 * 1024 * 1024).toLong()))
+    }
+
+    @Test
+    fun testFormatDuration_minutes() {
+        assertEquals("0m", formatDuration(0.0))
+        assertEquals("0m", formatDuration(45.0))
+        assertEquals("1m", formatDuration(60.0))
+        assertEquals("59m", formatDuration(3540.0))
+    }
+
+    @Test
+    fun testFormatDuration_hoursAndMinutes() {
+        assertEquals("1h 0m", formatDuration(3600.0))
+        assertEquals("1h 1m", formatDuration(3660.0))
+        assertEquals("23h 59m", formatDuration(86399.0))
+    }
+
+    @Test
+    fun testFormatDuration_daysHoursMinutes() {
+        assertEquals("1d 0h 0m", formatDuration(86400.0))
+        assertEquals("1d 1h 1m", formatDuration(90060.0))
+        assertEquals("3d 5h 30m", formatDuration(3 * 86400.0 + 5 * 3600.0 + 30 * 60.0))
+    }
+}