Keep overlay surfaces mapped so they open sharp (#13419)

The menu, emoji picker, clipboard, OSD, reminder flow and Wi-Fi QR code
mapped a fresh surface on every open. Qt drew its first frames before
Hyprland sent the surface's fractional scale: the pixel ratio stepped
2, 1, then 1.6, so on a 1.6 display each overlay showed blurry for
~350ms before going sharp.

A new OverlayWindow keeps the surface. Hidden, it parks as a 1x1,
input-less layer below windows, off the overlay layer so it never
blocks direct scanout. Showing only resizes and raises it, so the scale
is already settled. Content stays hidden until the surface has grown,
so no 1x1 frame is stretched across the screen, and the window follows
the focused monitor each time it is shown. The image picker's own
fullscreen parking moves onto it too.

Each parked overlay keeps a Qt window alive: at rest the shell holds
~40 MiB more RSS and ~20 MiB more GPU memory.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

shell/plugins/osd/Osd.qml

@@ -123,15 +123,11 @@ Item {
     function ping(): string { return "ok" }
   }
 
-  PanelWindow {
+  OverlayWindow {
     id: panel
-    visible: root.opened
-    anchors { top: true; bottom: true; left: true; right: true }
-    color: "transparent"
+    shown: root.opened
+    shownKeyboardFocus: WlrKeyboardFocus.None
     WlrLayershell.namespace: "omarchy-osd"
-    WlrLayershell.layer: WlrLayer.Overlay
-    WlrLayershell.keyboardFocus: WlrKeyboardFocus.None
-    exclusionMode: ExclusionMode.Ignore
     // Visual-only surface: keep the layer-shell input region empty so the OSD
     // never blocks clicks to the desktop below it.
     mask: Region {}