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/TopShelfSnapshotWriter.swift

@@ -100,7 +100,7 @@ private extension TopShelfSnapshotWriter {
             title: bookmark.title,
             authorName: bookmark.authorName,
             duration: bookmark.duration,
-            thumbnailURL: bookmark.thumbnailURLString,
+            thumbnailURL: reliableThumbnailURLString(bookmark.thumbnailURLString),
             deepLinkURL: deepLink,
             progressSeconds: nil
         )
@@ -118,7 +118,7 @@ private extension TopShelfSnapshotWriter {
             title: entry.title,
             authorName: entry.authorName,
             duration: entry.duration,
-            thumbnailURL: entry.thumbnailURLString,
+            thumbnailURL: reliableThumbnailURLString(entry.thumbnailURLString),
             deepLinkURL: deepLink,
             progressSeconds: entry.watchedSeconds
         )
@@ -190,8 +190,17 @@ private extension TopShelfSnapshotWriter {
         }
     }
 
+    /// The Top Shelf extension sets a single image URL with no failure fallback,
+    /// so rewrite to the always-available `hqdefault` variant: the best advertised
+    /// variant is often `maxresdefault`, which 404s for many older videos and
+    /// would leave an empty tile.
     static func bestThumbnailURL(from thumbnails: [Thumbnail]) -> String? {
-        thumbnails.max(by: { $0.quality < $1.quality })?.url.absoluteString
+        Thumbnail.reliableURL(for: thumbnails.max(by: { $0.quality < $1.quality })?.url)?.absoluteString
+    }
+
+    static func reliableThumbnailURLString(_ urlString: String?) -> String? {
+        guard let urlString else { return nil }
+        return Thumbnail.reliableURL(for: URL(string: urlString))?.absoluteString ?? urlString
     }
 }
 #endif