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/API/YatteeServerAPI.swift

@@ -1363,7 +1363,7 @@ private struct YatteePlaylist: Decodable, Sendable {
             description: description,
             author: authorId.map { Author(id: $0, name: author ?? "") },
             videoCount: videoCount,
-            thumbnailURL: validVideos.first?.thumbnails.first?.url,
+            thumbnailURL: Thumbnail.reliableURL(for: validVideos.first?.thumbnails.first?.url),
             videos: validVideos
         )
     }
@@ -1440,7 +1440,7 @@ private struct YatteeSearchPlaylist: Decodable, Sendable {
             title: title,
             author: authorId.map { Author(id: $0, name: author ?? "") },
             videoCount: videoCount,
-            thumbnailURL: thumbnailURL,
+            thumbnailURL: Thumbnail.reliableURL(for: thumbnailURL),
             videos: videos?.map { $0.toVideo() } ?? []
         )
     }