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.

Cable/Loads/LoadsView.swift

@@ -56,6 +56,10 @@ struct LoadsView: View {
         allChargers.filter { $0.system == system }
     }
 
+    private var hasComponents: Bool {
+        !savedLoads.isEmpty || !savedBatteries.isEmpty || !savedChargers.isEmpty
+    }
+
     var body: some View {
         VStack(spacing: 0) {
             TabView(selection: $selectedComponentTab) {
@@ -174,6 +178,7 @@ struct LoadsView: View {
                         } label: {
                             Image(systemName: "square.and.arrow.up")
                         }
+                        .disabled(!hasComponents)
                         .accessibilityIdentifier("system-overview-share-button")
                     }
                 } else if showPrimary || showEditLoads || showEditBatteries || showEditChargers {