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/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")
+    }
+}