feat(operations): Display average speed in performance graph

Change the performance graph marker to show average speed instead of current speed
from the last sample, providing a better overall view of transfer performance.

Changes:
- Calculate average speed across all samples using arithmetic mean
- Update string resource to workspace_operation_performance_avg_speed_label
- More accurately represents overall operation performance

This aligns with user expectations for performance metrics where average throughput
is more meaningful than instantaneous final speed.

Author: d4rken

app-workspace/src/main/java/eu/darken/butler/workspace/ui/operations/details/OperationPerformanceGraph.kt

@@ -210,13 +210,16 @@ fun OperationPerformanceGraph(
                 ),
             )
 
-            // Current speed label at last data point (Windows Explorer style)
-            val lastSample = performanceHistory.samples.lastOrNull()
-            if (lastSample != null) {
-                val currentSpeed = formatSpeed(lastSample.bytesPerSecond)
+            // Average speed label (Windows Explorer style)
+            if (performanceHistory.samples.isNotEmpty()) {
+                val averageSpeed = performanceHistory.samples
+                    .map { it.bytesPerSecond }
+                    .average()
+                    .toLong()
+                val formattedSpeed = formatSpeed(averageSpeed)
 
                 Text(
-                    text = stringResource(R.string.workspace_operation_performance_speed_label, currentSpeed),
+                    text = stringResource(R.string.workspace_operation_performance_avg_speed_label, formattedSpeed),
                     style = MaterialTheme.typography.labelMedium,
                     color = MaterialTheme.colorScheme.onSurface,
                     modifier = Modifier

app-workspace/src/main/res/values/strings.xml

@@ -142,5 +142,5 @@
 
     <!-- Performance Graph -->
     <string name="workspace_operation_performance_not_enough_data">Not enough data</string>
-    <string name="workspace_operation_performance_speed_label">Speed: %1$s</string>
+    <string name="workspace_operation_performance_avg_speed_label">Speed Ø: %1$s</string>
 </resources>