feat(sound): crossfade album art, fade to plain without it

The OSD background held one Image, so a track change swapped covers in a
single frame. Two images now take turns: the new cover loads into the
hidden one and is brought to the front only once it reports Ready, then
both fade over 300ms. Waiting for Ready is what keeps a slow or remote
art server from fading through black.

The blur, scrim, sheen and grain are grouped under one item whose
opacity follows artOn. It goes false when the player has no art or the
cover for the current track errors, so the panel fades back to its plain
translucent look instead of snapping, or keeping a stale cover that
reads as a fault. It goes true again only when a new cover has loaded,
so returning from plain never flashes the previous one.

The back image's source is cleared before being set, so a URL it
already holds still reloads and reports a status. onWantChanged misses
the value at load, hence the Component.onCompleted call.

Not yet seen on screen: the shell reloaded it cleanly but the fade was
not watched through a track change.

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

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