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.

CableUITestsScreenshot/SystemExportButtonUITests.swift

@@ -0,0 +1,71 @@
+import XCTest
+
+/// Verifies that the Overview export/share menu is only available once the
+/// system actually contains something worth exporting.
+final class SystemExportButtonUITests: XCTestCase {
+
+    override func setUpWithError() throws {
+        try super.setUpWithError()
+        continueAfterFailure = false
+        XCUIDevice.shared.orientation = .portrait
+    }
+
+    @MainActor
+    func testExportIsDisabledForSystemWithoutComponents() throws {
+        let app = launch(arguments: ["--uitest-reset-data"])
+
+        let createSystemButton = app.buttons["create-system-button"]
+        XCTAssertTrue(createSystemButton.waitForExistence(timeout: 15))
+        createSystemButton.tap()
+
+        let shareButton = app.buttons["system-overview-share-button"]
+        XCTAssertTrue(shareButton.waitForExistence(timeout: 15))
+        XCTAssertFalse(
+            shareButton.isEnabled,
+            "Export must stay disabled while the system has no loads, batteries or chargers"
+        )
+    }
+
+    @MainActor
+    func testExportIsEnabledOnceComponentsExist() throws {
+        let app = launch(arguments: ["--uitest-reset-data", "--uitest-sample-data"])
+
+        openFirstSystem(in: app)
+
+        let shareButton = app.buttons["system-overview-share-button"]
+        XCTAssertTrue(shareButton.waitForExistence(timeout: 15))
+        XCTAssertTrue(
+            shareButton.isEnabled,
+            "Export must be available for a system that has components"
+        )
+    }
+
+    // MARK: - Helpers
+
+    private func launch(arguments: [String]) -> XCUIApplication {
+        let app = XCUIApplication()
+        app.launchArguments = arguments
+        app.launch()
+        return app
+    }
+
+    private func openFirstSystem(in app: XCUIApplication) {
+        let list: XCUIElement
+        if app.collectionViews["systems-list"].waitForExistence(timeout: 15) {
+            list = app.collectionViews["systems-list"]
+        } else {
+            list = app.collectionViews.firstMatch
+        }
+        XCTAssertTrue(list.waitForExistence(timeout: 15))
+
+        let firstCell = list.cells.element(boundBy: 0)
+        XCTAssertTrue(firstCell.waitForExistence(timeout: 10))
+
+        let cellButton = firstCell.buttons.firstMatch
+        if cellButton.exists {
+            cellButton.tap()
+        } else {
+            firstCell.coordinate(withNormalizedOffset: CGVector(dx: 0.5, dy: 0.5)).tap()
+        }
+    }
+}