Apply thumbnail fallback everywhere a single URL was rendered

The previous fix rebuilt the quality chain when converting persisted
models back to videos, but many consumers discarded it again by
rendering bestThumbnail (usually a maxresdefault.jpg that 404s for
older videos) as a single URL with no fallback.

Extract the retry logic from VideoThumbnailView into FallbackLazyImage
and use it at every in-app site that can iterate: player thumbnails
(mini bar, expanded sheet loaders, autoplay previews, tvOS audio-mode
artwork - previously a silent black screen), video info card and tvOS
header, and the tvOS playlist cover.

Sites that fetch or send exactly one URL are rewritten to the
always-available hqdefault variant via Thumbnail.reliableURL (exposed
as Video.reliableThumbnailURL): Now Playing artwork, Top Shelf
snapshots, remote control state, the frozen transition thumbnail,
blurred info background, navigation covers, and playlist covers
derived from a video's first thumbnail in Invidious/Yattee Server
responses.

Also invert RecentPlaylist's upgrade helper, which rewrote covers *to*
maxresdefault, walk the quality chain when caching download thumbnails
for offline artwork instead of giving up after one 404, and expand the
remaining single-thumbnail Piped conversions into full chains.

Yattee/Services/Player/PlayerService.swift

@@ -522,8 +522,10 @@ final class PlayerService {
                    let localThumbnailPath = download.localThumbnailPath {
                     localThumbnailURL = downloadManager.downloadsDirectory().appendingPathComponent(localThumbnailPath)
                 }
+                // Reliable (hqdefault) variant: artwork is a single fetch with no
+                // fallback, and the best advertised variant often 404s.
                 await nowPlayingService.loadArtwork(
-                    from: videoForNowPlaying.bestThumbnail?.url,
+                    from: videoForNowPlaying.reliableThumbnailURL,
                     localPath: localThumbnailURL
                 )
             }