feat: swap translation languages with Return on empty input

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