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>

test/shell.d/image-picker-test.sh

@@ -70,14 +70,8 @@ assert(
   'image picker leaves theme mode when another caller opens it'
 )
 assert(
-  /PanelWindow \{[\s\S]*?visible: true[\s\S]*?mask: root\.opened \? null : closedMask[\s\S]*?WlrLayershell\.layer: root\.opened \? WlrLayer\.Overlay : WlrLayer\.Bottom/.test(imagePickerQml) &&
-    /Region \{ id: closedMask \}/.test(imagePickerQml),
-  'image picker keeps its surface mapped, parked input-less below windows while closed'
-)
-assert(
-  /function openSelector[\s\S]*?targetScreen = focusedScreen\(\) \|\| targetScreen/.test(imagePickerQml) &&
-    /screen: root\.targetScreen/.test(imagePickerQml),
-  'image picker follows the focused monitor on each open'
+  /OverlayWindow \{\s*id: panel\s*shown: root\.opened\s*shownKeyboardFocus: root\.imagesLoaded \? WlrKeyboardFocus\.Exclusive : WlrKeyboardFocus\.None/.test(imagePickerQml),
+  'image picker parks on OverlayWindow and takes the keyboard once images load'
 )
 assert(
   /source: item\.sourceActivated && item\.thumbnailPath \? Util\.fileUrl\(item\.thumbnailPath\) : ""[\s\S]*asynchronous: false/.test(imagePickerQml),