Float as a KWin-managed window instead of a layer surface

The grab bar didn't move the window by mouse, and the corner grip never
resized it (the bar's handler took over its drag). The bar and grip now
start the window manager's move/resize, and Super+LMB/RMB work too.

A KWin rule in kwinrulesrc keeps the window above and unfocused, off the
taskbar, pager and switcher, and remembers its position. KWin now places
the window and keeps it on screen.

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

qml/ResizeHandle.qml

@@ -1,11 +1,10 @@
 import QtQuick
 
-// Corner grip. The window grows from its top-left corner, so window
-// coordinates stay put while resizing: report each step's delta.
+// Corner grip. Resizing is up to the owner (the window manager's resize).
 Item {
     id: grip
 
-    signal resized(real dw, real dh)
+    signal dragStarted()
 
     implicitWidth: 28
     implicitHeight: 28
@@ -28,17 +27,10 @@ Item {
     }
 
     DragHandler {
-        property point last
-
         target: null
+        // The grab bar lies underneath and would take the drag over.
+        grabPermissions: PointerHandler.TakeOverForbidden
         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
-        }
+        onActiveChanged: if (active) grip.dragStarted()
     }
 }