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.

default/themed/shell.toml.tpl

@@ -224,12 +224,10 @@ selection        = "{{ accent }}"
 selection-alpha  = 0.45
 
 [image-picker]
-# Carousel-style picker. The picker has no card surface, so `scrim` is
-# the full-screen wash. Per-slice dim overlays and text outlines on top
-# of the scrim track the foundational background color directly.
+# Carousel-style picker shown directly over the desktop, with no card
+# surface or backdrop wash. Per-slice dim overlays and text outlines
+# track the foundational background color directly.
 # unselected-border-alpha softens carousel slices that aren't selected.
-scrim                   = "{{ background }}"
-scrim-alpha             = 0.5
 text                    = "{{ foreground }}"
 selected-border         = "{{ accent }}"
 selected-border-alpha   = 1.0