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>

src/app/DBusControl.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <QObject>
+
+class VisibilityController;
+
+// Session-bus remote control, e.g. for a global shortcut:
+//   qdbus org.sloposk.Keyboard /Keyboard org.sloposk.Keyboard.Toggle
+class DBusControl : public QObject
+{
+    Q_OBJECT
+    Q_CLASSINFO("D-Bus Interface", "org.sloposk.Keyboard")
+
+public:
+    explicit DBusControl(VisibilityController &visibility, QObject *parent = nullptr);
+
+    // False if the bus is unavailable or another instance owns the name.
+    bool registerOnSessionBus();
+
+public Q_SLOTS:
+    Q_SCRIPTABLE void Show();
+    Q_SCRIPTABLE void Hide();
+    Q_SCRIPTABLE void Toggle();
+
+private:
+    VisibilityController &m_visibility;
+};