window-switcher: show the previews over the wallpaper, faded in

The full-screen dim over the live windows is replaced by DP-1's own
wallpaper, read from ~/.config/wallp/wall_h on each open and drawn
opaque, so the previews sit on what looks like an empty desktop the way
Mission Control does it. A light 40% Theme.base layer stays on top of
it, because a bright wallpaper at full strength glared behind the
previews.

The overlay fades in over 220ms instead of appearing at once. The fade
waits for the image to decode (or to fail, or for there to be no
wallp file), otherwise Theme.base would flash first. Opacity is
derived from the image status rather than starting at 1, so the
initial Null status cannot play a 1-to-0 animation. Closing is still
instant: fading out would keep the layer surface alive past the 60ms
focus timer.

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

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