Show the image picker instantly, with no backdrop (#14709)

Remove the carousel's fade and geometry transitions, hold the reveal until every visible preview has decoded and two frames have pre-rendered (falling back to revealing anyway after 400ms), and drop the fullscreen scrim and its theme tokens so the picker sits directly on the desktop. Only Escape is accepted before the picker is visible.

shell/Commons/Color.qml

@@ -128,11 +128,9 @@ QtObject {
     property color borderError: root.composed("lock.border-error", "lock.border-alpha", root.urgent, 1.0)
     property color selection: root.composed("lock.selection", "lock.selection-alpha", root.accent, 0.45)
   }
-  // The image picker has no card surface; `scrim` is the full-screen dim
-  // wash, and per-slice dim overlays / text outlines use the foundational
-  // `background` color directly.
+  // The image picker has no card surface or backdrop; per-slice dim
+  // overlays / text outlines use the foundational `background` color directly.
   readonly property QtObject imagePicker: QtObject {
-    property color scrim: root.composed("image-picker.scrim", "image-picker.scrim-alpha", root.background, 0.5)
     property color text: root.pick("image-picker.text", root.foreground)
     property color selectedBorder: root.composed("image-picker.selected-border", "image-picker.selected-border-alpha", root.accent, 1.0)
     property color unselectedBorder: root.composed("image-picker.unselected-border", "image-picker.unselected-border-alpha", root.foreground, 0.28)