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>

qml/ResizeHandle.qml

@@ -0,0 +1,44 @@
+import QtQuick
+
+// Corner grip. The window grows from its top-left corner, so window
+// coordinates stay put while resizing: report each step's delta.
+Item {
+    id: grip
+
+    signal resized(real dw, real dh)
+
+    implicitWidth: 28
+    implicitHeight: 28
+
+    // Two strokes across the corner, each from (w - d, h) to (w, h - d).
+    Repeater {
+        model: [9, 16]
+        Rectangle {
+            required property real modelData
+            readonly property real inset: 4
+
+            width: modelData * Math.SQRT2
+            height: 2
+            radius: 1
+            color: Theme.handle
+            rotation: -45
+            x: grip.width - inset - modelData / 2 - width / 2
+            y: grip.height - inset - modelData / 2 - height / 2
+        }
+    }
+
+    DragHandler {
+        property point last
+
+        target: null
+        cursorShape: Qt.SizeFDiagCursor
+        onActiveChanged: last = centroid.scenePressPosition
+        onCentroidChanged: {
+            if (!active)
+                return
+            const p = centroid.scenePosition
+            grip.resized(p.x - last.x, p.y - last.y)
+            last = p
+        }
+    }
+}