feat: swap translation languages with Return on empty input

Sources/Conjure/TranslationController.swift

@@ -239,8 +239,11 @@ final class TranslationController: NSViewController {
     func inputChanged() {
         cancelRequest()
         output.string = ""
-        translateButton.title = "Translate"
-        translateButton.toolTip = "Translate the input (Return). Escape dismisses Conjure."
+        let isEmpty = (input?() ?? "").trimmingCharacters(in: .whitespacesAndNewlines).isEmpty
+        translateButton.title = isEmpty ? "Swap languages" : "Translate"
+        translateButton.toolTip = isEmpty
+            ? "Swap source and target languages (Return)."
+            : "Translate the input (Return). Escape dismisses Conjure."
         settingsButton.isHidden = true
         languageButton.title = "\(languageName(sourceLanguage)) → \(languageName(targetLanguage))"
         output.setAccessibilityLabel("\(languageName(targetLanguage)) translation")
@@ -269,6 +272,10 @@ final class TranslationController: NSViewController {
 
     @objc func performPrimaryAction() {
         guard task == nil else { return }
+        if (input?() ?? "").trimmingCharacters(in: .whitespacesAndNewlines).isEmpty {
+            swapLanguages()
+            return
+        }
         guard !output.string.isEmpty else { translate(); return }
         pasteboard.clearContents()
         if pasteboard.setString(output.string, forType: .string) {