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/Views/Playlist/UnifiedPlaylistDetailView.swift

@@ -107,7 +107,9 @@ struct UnifiedPlaylistDetailView: View {
     }
 
     private var navigationThumbnailURL: URL? {
-        videos.first?.bestThumbnail?.url ?? thumbnailURL ?? cachedHeader?.thumbnailURL
+        // Rendered as a single URL without fallback, so rewrite to the
+        // always-available hqdefault variant.
+        Thumbnail.reliableURL(for: videos.first?.bestThumbnail?.url ?? thumbnailURL ?? cachedHeader?.thumbnailURL)
     }
 
     /// Summary text for the playlist (e.g., "5 videos · 1h 23m").
@@ -532,8 +534,9 @@ struct UnifiedPlaylistDetailView: View {
 
     @ViewBuilder
     private var tvOSPlaylistThumbnail: some View {
-        let url = videos.first?.bestThumbnail?.url ?? thumbnailURL
-        LazyImage(url: url) { state in
+        let urls = (videos.first?.thumbnailURLsByQuality ?? [])
+            + [Thumbnail.reliableURL(for: thumbnailURL)].compactMap { $0 }
+        FallbackLazyImage(urls: urls) { state in
             if let image = state.image {
                 image
                     .resizable()