Fall back to lower-quality thumbnails when higher-res variants 404

YouTube's CDN advertises maxres/sddefault thumbnails for every video but
only generates them for sufficiently high-res uploads, so older/low-res
videos return 404 for those qualities. The view picked the best quality
and showed a blank placeholder on failure with no fallback.

VideoThumbnailView now accepts an ordered fallback chain and advances to
the next candidate via NukeUI's onCompletion when a load fails, so a valid
thumbnail is always shown. DeArrowVideoThumbnail feeds it DeArrow branding
first, then the video's own thumbnails best-quality-first.

Yattee/Models/Video.swift

@@ -91,6 +91,15 @@ struct Video: Identifiable, Codable, Sendable {
         thumbnails.sorted { $0.quality > $1.quality }.first
     }
 
+    /// Thumbnail URLs ordered best-quality-first.
+    ///
+    /// Used as a fallback chain: backends (and YouTube's CDN) often advertise
+    /// `maxres`/`sddefault` variants that don't actually exist for a given video
+    /// and 404. Consumers try these in order and drop to the next when one fails.
+    var thumbnailURLsByQuality: [URL] {
+        thumbnails.sorted { $0.quality > $1.quality }.map(\.url)
+    }
+
     var formattedDuration: String {
         guard !isLive else { return "LIVE" }
         guard duration > 0 else { return "" }

Yattee/Views/Components/DeArrowVideoThumbnail.swift

@@ -34,8 +34,15 @@ struct DeArrowVideoThumbnail: View {
         appEnvironment?.deArrowBrandingProvider
     }
 
-    private var displayThumbnailURL: URL? {
-        deArrowProvider?.thumbnailURL(for: video) ?? video.bestThumbnail?.url
+    /// Ordered thumbnail candidates: DeArrow branding (if any) first, then the
+    /// video's own thumbnails best-quality-first. The view falls back through
+    /// these so a thumbnail always shows even when higher-res variants 404.
+    private var thumbnailCandidates: (primary: URL?, fallbacks: [URL]) {
+        let videoURLs = video.thumbnailURLsByQuality
+        if let deArrowURL = deArrowProvider?.thumbnailURL(for: video) {
+            return (deArrowURL, videoURLs)
+        }
+        return (videoURLs.first, Array(videoURLs.dropFirst()))
     }
 
     #if !os(tvOS)
@@ -91,7 +98,8 @@ struct DeArrowVideoThumbnail: View {
 
     var body: some View {
         VideoThumbnailView(
-            url: displayThumbnailURL,
+            url: thumbnailCandidates.primary,
+            fallbackURLs: thumbnailCandidates.fallbacks,
             cornerRadius: cornerRadius,
             watchProgress: watchProgress,
             duration: duration,

Yattee/Views/Components/VideoThumbnailView.swift

@@ -18,6 +18,14 @@ import NukeUI
 struct VideoThumbnailView: View {
     let url: URL?
 
+    /// Lower-quality URLs to try, in priority order, if `url` fails to load.
+    ///
+    /// YouTube's CDN (and the backends that wrap it) frequently advertise
+    /// `maxresdefault`/`sddefault` thumbnails that don't exist for older or
+    /// low-resolution uploads and return 404. When the primary `url` fails we
+    /// step through these so a valid thumbnail is always shown.
+    var fallbackURLs: [URL] = []
+
     var cornerRadius: CGFloat = 8
     var watchProgress: Double? = nil
     var duration: String? = nil
@@ -30,8 +38,23 @@ struct VideoThumbnailView: View {
     var placeholderTitle: String? = nil
     var isWatched: Bool = false
 
+    /// Index into `candidates` of the URL currently being attempted.
+    @State private var candidateIndex = 0
+
+    /// De-duplicated candidate URLs, best-quality first.
+    private var candidates: [URL] {
+        var seen = Set<URL>()
+        return ([url] + fallbackURLs).compactMap { $0 }.filter { seen.insert($0).inserted }
+    }
+
+    /// The URL currently being shown, advancing through `candidates` on failure.
+    private var currentURL: URL? {
+        guard candidates.indices.contains(candidateIndex) else { return candidates.last }
+        return candidates[candidateIndex]
+    }
+
     var body: some View {
-        LazyImage(url: url) { state in
+        LazyImage(url: currentURL) { state in
             if let image = state.image {
                 image
                     .resizable()
@@ -52,6 +75,13 @@ struct VideoThumbnailView: View {
                     }
             }
         }
+        .onCompletion { result in
+            // On a failed load, fall back to the next (lower-quality) candidate.
+            if case .failure = result, candidateIndex < candidates.count - 1 {
+                candidateIndex += 1
+            }
+        }
+        .onChange(of: candidates) { candidateIndex = 0 }
         .aspectRatio(16/9, contentMode: .fit)
         .overlay(alignment: .bottom) {
             watchProgressBar