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/MiniPlayerView.swift

@@ -67,12 +67,11 @@ struct MiniPlayerView: View {
         return currentVideo?.title ?? String(localized: "player.notPlaying")
     }
 
-    /// The thumbnail URL to display, preferring DeArrow thumbnail if available.
-    private var displayThumbnailURL: URL? {
-        if let video = currentVideo, let deArrowThumbnail = deArrowProvider?.thumbnailURL(for: video) {
-            return deArrowThumbnail
-        }
-        return currentVideo?.bestThumbnail?.url
+    /// Thumbnail URLs to try in order, preferring DeArrow, then the quality chain.
+    private var displayThumbnailURLs: [URL] {
+        guard let video = currentVideo else { return [] }
+        let deArrowThumbnail = deArrowProvider?.thumbnailURL(for: video)
+        return [deArrowThumbnail].compactMap { $0 } + video.thumbnailURLsByQuality
     }
 
     // MARK: - Actions
@@ -406,7 +405,7 @@ struct MiniPlayerView: View {
 
     @ViewBuilder
     private var thumbnailView: some View {
-        LazyImage(url: displayThumbnailURL) { state in
+        FallbackLazyImage(urls: displayThumbnailURLs) { state in
             if let image = state.image {
                 image
                     .resizable()