Floating polish: visibility, resize, saved geometry, D-Bus, desktop entry

- VisibilityController: show on field focus, dismiss until next focus,
  manual show/hide/toggle overrides
- FloatingWindow: resize from the corner grip, clamp to screen
- WindowSettings: geometry per screen, debounced saves
- DBusControl: org.sloposk.Keyboard Show/Hide/Toggle for global shortcuts
- org.sloposk.keyboard.desktop so System Settings lists it as a virtual keyboard
- README with build/run/layout docs

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

tests/tst_visibility.cpp

@@ -0,0 +1,48 @@
+#include "window/VisibilityController.h"
+
+#include <QTest>
+
+class TestVisibility : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void followsFieldFocus()
+    {
+        VisibilityController v;
+        v.setFieldActive(true);
+        QVERIFY(v.isVisible());
+        v.setFieldActive(false);
+        QVERIFY(!v.isVisible());
+    }
+
+    void dismissLastsUntilFocusChanges()
+    {
+        VisibilityController v;
+        v.setFieldActive(true);
+        v.hide();
+        QVERIFY(!v.isVisible());
+        v.setFieldActive(false);
+        v.setFieldActive(true);
+        QVERIFY(v.isVisible());
+    }
+
+    void toggleShowsWithoutField()
+    {
+        VisibilityController v;
+        v.toggle();
+        QVERIFY(v.isVisible());
+        v.toggle();
+        QVERIFY(!v.isVisible());
+    }
+
+    void standaloneAlwaysVisible()
+    {
+        VisibilityController v;
+        v.setStandalone(true);
+        QVERIFY(v.isVisible());
+    }
+};
+
+QTEST_GUILESS_MAIN(TestVisibility)
+#include "tst_visibility.moc"