feat(android): generated placeholder art and a skeleton library

A title without artwork shows the accent as a gradient with its initials
instead of a black card with a play triangle, so a library of unmatched
titles is no longer a grid of identical cards. Art with the name already
over it, and episode stills, get the gradient without initials. While
artwork loads, and while the catalog first loads, the page shows pulsing
shapes where the posters will be rather than a spinner.

The screenshot tests turn artwork loading off, so whether a failed fetch
lands before the capture no longer decides what the image shows.

android/app/src/main/java/io/narl/protonstream/playback/PlayerScreen.kt

@@ -954,6 +954,7 @@ private fun EpisodeChooser(
                             episode.label,
                             Modifier.fillMaxSize(),
                             fallback = episode.thumbnailSource,
+                            labelled = false,
                         )
                         episode.progress?.takeIf { !episode.watched && !current }?.let { progress ->
                             AccentProgress(
@@ -1048,6 +1049,7 @@ private fun UpNextCard(
                     upNext.label,
                     Modifier.fillMaxSize(),
                     fallback = upNext.thumbnailSource,
+                    labelled = false,
                 )
                 Box(Modifier.size(40.dp).clip(CircleShape).background(Color.Black.copy(alpha = 0.5f)))
                 CircularProgressIndicator(