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 "" }