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/Player/tvOS/TVAutoplayCountdownView.swift

@@ -62,7 +62,7 @@ struct TVAutoplayCountdownView: View {
     private var nextVideoCard: some View {
         HStack(spacing: 20) {
             // Thumbnail
-            LazyImage(url: nextVideo.video.bestThumbnail?.url) { state in
+            FallbackLazyImage(urls: nextVideo.video.thumbnailURLsByQuality) { state in
                 if let image = state.image {
                     image
                         .resizable()

Yattee/Views/Player/tvOS/TVPlayerView.swift

@@ -6,6 +6,7 @@
 //
 
 #if os(tvOS)
+import NukeUI
 import SwiftUI
 
 /// Focus targets for tvOS player controls navigation.
@@ -644,13 +645,15 @@ struct TVPlayerView: View {
             // Thumbnail for audio-only playback and the pre-backend loading state
             if isAudioOnly || !hasBackend,
                let video = playerState?.currentVideo,
-               let thumbnailURL = video.bestThumbnail?.url {
-                AsyncImage(url: thumbnailURL) { image in
-                    image
-                        .resizable()
-                        .aspectRatio(contentMode: .fit)
-                } placeholder: {
-                    Color.black
+               !video.thumbnailURLsByQuality.isEmpty {
+                FallbackLazyImage(urls: video.thumbnailURLsByQuality) { state in
+                    if let image = state.image {
+                        image
+                            .resizable()
+                            .aspectRatio(contentMode: .fit)
+                    } else {
+                        Color.black
+                    }
                 }
                 .allowsHitTesting(false)
             }