Disable export until a system has components

The Overview share menu offered a wiring diagram and a full report even
for an empty system. Gate it on loads, batteries or chargers being
present (iOS + Android) and add UI tests for both states.

android/app/src/main/java/app/voltplan/cable/ui/system/SystemDetailScreen.kt

@@ -157,7 +157,7 @@ fun SystemDetailScreen(
                                     strokeWidth = 2.dp,
                                 )
                             } else {
-                                IconButton(onClick = { showOverviewMenu = true }) {
+                                IconButton(onClick = { showOverviewMenu = true }, enabled = state.hasComponents) {
                                     Icon(Icons.Outlined.IosShare, contentDescription = stringResource(R.string.overview_share_pdf))
                                 }
                             }

android/app/src/main/java/app/voltplan/cable/ui/system/SystemDetailViewModel.kt

@@ -22,6 +22,9 @@ data class DetailState(
     val chargers: List<SavedCharger> = emptyList(),
 ) {
     val metrics: SystemMetrics get() = SystemMetrics(loads, batteries, chargers)
+
+    val hasComponents: Boolean
+        get() = loads.isNotEmpty() || batteries.isNotEmpty() || chargers.isNotEmpty()
 }
 
 class SystemDetailViewModel(