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/PipedAPI.swift

@@ -532,9 +532,7 @@ private struct PipedVideo: Decodable, Sendable {
             publishedText: uploadedDate,
             viewCount: views.map { Int($0) },
             likeCount: nil,
-            thumbnails: thumbnail.flatMap { URL(string: $0) }.map {
-                [Thumbnail(url: $0, quality: .high)]
-            } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: thumbnail.flatMap { URL(string: $0) }),
             isLive: duration == -1,
             isUpcoming: false,
             scheduledStartTime: nil
@@ -771,9 +769,7 @@ private struct PipedSearchItem: Decodable, Sendable {
             publishedText: uploadedDate,
             viewCount: views.map { Int($0) },
             likeCount: nil,
-            thumbnails: thumbnail.flatMap { URL(string: $0) }.map {
-                [Thumbnail(url: $0, quality: .high)]
-            } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: thumbnail.flatMap { URL(string: $0) }),
             isLive: duration == -1,
             isUpcoming: false,
             scheduledStartTime: nil