Keep the window alive on close; add a system toggle to VisibilityController

- Main.qml: ignore close requests (Alt+F4, task manager), so the window
  can't be hidden without VisibilityController knowing.
- main.cpp: don't quit when the last window closes; KWin only restarts
  the input method after a crash.
- VisibilityController::setEnabled(): when false, the keyboard is hidden
  regardless of other state; changing it clears show()/hide() overrides.
  Not wired to KWin's tray toggle yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/tst_visibility.cpp

@@ -70,6 +70,19 @@ private Q_SLOTS:
         QVERIFY(!v.isVisible());
     }
 
+    void systemToggleGatesEverything()
+    {
+        VisibilityController v;
+        v.setAlwaysVisible(true);
+        v.setEnabled(false);
+        QVERIFY(!v.isVisible());
+        v.show();
+        QVERIFY(!v.isVisible());
+        v.hide();
+        v.setEnabled(true); // turning it back on forgets the dismiss
+        QVERIFY(v.isVisible());
+    }
+
     void standaloneAlwaysVisible()
     {
         VisibilityController v;