feat: swap translation languages with Return on empty input

README.md

@@ -136,9 +136,9 @@ Notes autosave locally as RTFD packages, including embedded images, under `~/Lib
 
 ## Translation
 
-Search for **Conjure Translate**, identified by its centered 🌏 icon, and open it. Choose a provider in the pill-shaped header selector, enter text in the search bar, then press Return or click **Translate**. Typing alone never sends text. When the translation is available, the action changes to **Copy**: press Return or click it to copy the full translation and close Conjure. Command-C still copies selected output. Editing the input restores **Translate**.
+Search for **Conjure Translate**, identified by its centered 🌏 icon, and open it. With empty or whitespace-only input, Return or **Swap languages** reverses the source and target without submitting a request. Enter text to change the action to **Translate**; press Return or click it to submit. Typing alone never sends text. When the translation is available, the action changes to **Copy**: press Return or click it to copy the full translation and close Conjure. Command-C still copies selected output. Editing the input clears the result and restores **Translate**, or **Swap languages** if empty. The action is disabled while translating.
 
-The footer places **Translate/Copy** and its Return keycap before **Actions**, whose Command and K keys use separate keycaps, matching the launcher.
+The footer places the current primary action and its Return keycap before **Actions**, whose Command and K keys use separate keycaps, matching the launcher.
 
 Press Command-P or click the provider selector to expand a searchable action-style popup from its top edge downwards, hiding the indicator until the popup closes. A checkmark marks the current provider. Use Up/Down and Return or click a provider to select it; Escape, Back, or clicking outside closes the popup. Changing providers clears the previous result without submitting a request.
 

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) {

Tests/ConjureKitTests/TranslationExecutionTests.swift

@@ -58,6 +58,26 @@ private func descendants(_ view: NSView) -> [NSView] { [view] + view.subviews.fl
 
 @Suite(.serialized)
 struct TranslationExecutionTests {
+@Test @MainActor
+func emptyPrimaryActionSwapsLanguagesWithoutSubmitting() {
+    _ = NSApplication.shared
+    let suite = "TranslateEmpty.\(UUID().uuidString)"
+    let defaults = UserDefaults(suiteName: suite)!
+    defer { defaults.removePersistentDomain(forName: suite) }
+    let tool = TranslationController(defaults: defaults,
+        myMemory: { _, _, _ in Issue.record("Empty input must not submit"); return "unexpected" })
+    tool.input = { " \n\t" }
+    _ = tool.view
+    let button = descendants(tool.view).compactMap { $0 as? NSButton }
+        .first { $0.action == NSSelectorFromString("performPrimaryAction") }!
+    button.performClick(nil)
+    #expect(tool.sourceLanguage == "ja")
+    #expect(tool.targetLanguage == "en")
+    tool.performPrimaryAction()
+    #expect(tool.sourceLanguage == "en")
+    #expect(tool.targetLanguage == "ja")
+}
+
 @Test @MainActor
 func languageActionsInvalidateOldResultsAndApplyTheSelectedPair() async throws {
     _ = NSApplication.shared