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/Models/Video.swift

@@ -100,6 +100,16 @@ struct Video: Identifiable, Codable, Sendable {
         thumbnails.sorted { $0.quality > $1.quality }.map(\.url)
     }
 
+    /// Best thumbnail URL rewritten to the always-available `hqdefault` variant.
+    ///
+    /// For consumers that render or fetch a single URL with no way to fall back
+    /// through the quality chain (Now Playing artwork, Top Shelf, remote control,
+    /// navigation covers): the best advertised variant is often `maxresdefault`,
+    /// which 404s for many older videos.
+    var reliableThumbnailURL: URL? {
+        Thumbnail.reliableURL(for: bestThumbnail?.url)
+    }
+
     var formattedDuration: String {
         guard !isLive else { return "LIVE" }
         guard duration > 0 else { return "" }