Fix missing thumbnails for videos saved to library

Persistence models (local playlists, watch history, bookmarks,
downloads) store only the best advertised thumbnail URL - usually
maxresdefault.jpg, which doesn't exist for many older videos and 404s.
Live API results survive this because views fall back through the full
quality chain, but toVideo() rebuilt videos with that single dead URL,
leaving placeholder covers.

Reconstruct the quality fallback chain from the stored YouTube-style
URL at read time (Thumbnail.fallbackChain), and rewrite playlist list
covers to the always-available hqdefault variant
(Thumbnail.reliableURL), since those views render a single URL without
fallback. Also expand the fabricated maxres URL in PipedAPI into a
full chain.

Yattee/Data/Bookmark.swift

@@ -217,7 +217,7 @@ final class Bookmark {
             publishedText: publishedText,
             viewCount: viewCount,
             likeCount: nil,
-            thumbnails: thumbnailURL.map { [Thumbnail(url: $0, width: nil, height: nil)] } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: thumbnailURL),
             isLive: isLive,
             isUpcoming: false,
             scheduledStartTime: nil

Yattee/Data/LocalPlaylist.swift

@@ -75,10 +75,16 @@ final class LocalPlaylist {
     }
 
     /// The first video's thumbnail URL for display.
+    ///
+    /// Rewritten to the always-available `hqdefault` variant because the stored
+    /// URL is the best advertised quality (often `maxresdefault`), which 404s
+    /// for many older videos and would leave the cover blank.
     var thumbnailURL: URL? {
-        (items ?? []).sorted { $0.sortOrder < $1.sortOrder }
+        Thumbnail.reliableURL(
+            for: (items ?? []).sorted { $0.sortOrder < $1.sortOrder }
                 .first?
                 .thumbnailURL
+        )
     }
 
     /// Sorted items by order.
@@ -249,7 +255,7 @@ extension LocalPlaylistItem {
             publishedText: nil,
             viewCount: nil,
             likeCount: nil,
-            thumbnails: thumbnailURL.map { [Thumbnail(url: $0, quality: .medium)] } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: thumbnailURL),
             isLive: isLive,
             isUpcoming: false,
             scheduledStartTime: nil

Yattee/Data/WatchEntry.swift

@@ -241,7 +241,7 @@ extension WatchEntry {
             publishedText: nil,
             viewCount: nil,
             likeCount: nil,
-            thumbnails: thumbnailURL.map { [Thumbnail(url: $0, quality: .medium)] } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: thumbnailURL),
             isLive: isLive,
             isUpcoming: false,
             scheduledStartTime: nil

Yattee/Models/Video.swift

@@ -311,6 +311,55 @@ struct Thumbnail: Codable, Hashable, Sendable {
         self.width = width
         self.height = height
     }
+
+    /// Expands a single stored YouTube-style thumbnail URL (`/vi/<id>/<variant>.jpg`)
+    /// into a best-first fallback chain. Persistence models (playlists, watch history,
+    /// bookmarks) store only the best advertised URL — usually `maxresdefault.jpg`,
+    /// which doesn't exist for many older videos and 404s. Reconstructing the
+    /// lower-quality variants lets thumbnail views fall back the same way they do
+    /// for live API results. Non-matching URLs get a single-entry chain.
+    /// Rewrites a YouTube-style thumbnail URL (`/vi/<id>/<variant>.jpg`) to the
+    /// `hqdefault.jpg` variant, which exists for effectively every video (unlike
+    /// `maxresdefault`/`sddefault`, which 404 for many older uploads). Use where a
+    /// single URL is displayed without fallback, e.g. playlist covers. Non-matching
+    /// URLs are returned unchanged.
+    static func reliableURL(for url: URL?) -> URL? {
+        guard let url else { return nil }
+        let variants = ["maxresdefault.jpg", "sddefault.jpg", "hqdefault.jpg", "mqdefault.jpg", "default.jpg"]
+        let path = url.path
+        guard path.range(of: #"/vi/[^/]+/"#, options: .regularExpression) != nil,
+              let match = variants.first(where: { path.hasSuffix($0) }),
+              var components = URLComponents(url: url, resolvingAgainstBaseURL: false) else {
+            return url
+        }
+        components.path = String(path.dropLast(match.count)) + "hqdefault.jpg"
+        return components.url ?? url
+    }
+
+    static func fallbackChain(for url: URL?) -> [Thumbnail] {
+        guard let url else { return [] }
+        let variants: [(suffix: String, quality: Quality)] = [
+            ("maxresdefault.jpg", .maxres),
+            ("sddefault.jpg", .standard),
+            ("hqdefault.jpg", .high),
+            ("mqdefault.jpg", .medium),
+            ("default.jpg", .default),
+        ]
+        let path = url.path
+        guard path.range(of: #"/vi/[^/]+/"#, options: .regularExpression) != nil,
+              let current = variants.first(where: { path.hasSuffix($0.suffix) }),
+              let components = URLComponents(url: url, resolvingAgainstBaseURL: false) else {
+            return [Thumbnail(url: url, quality: .medium)]
+        }
+        let basePath = String(path.dropLast(current.suffix.count))
+        return variants.compactMap { variant in
+            guard variant.quality <= current.quality else { return nil }
+            var variantComponents = components
+            variantComponents.path = basePath + variant.suffix
+            guard let variantURL = variantComponents.url else { return nil }
+            return Thumbnail(url: variantURL, quality: variant.quality)
+        }
+    }
 }
 
 // MARK: - Preview Support

Yattee/Services/API/PipedAPI.swift

@@ -590,7 +590,7 @@ private struct PipedStreamResponse: Decodable, Sendable {
         let thumbnails: [Thumbnail] = {
             if !resolvedVideoId.isEmpty,
                let url = URL(string: "https://i.ytimg.com/vi/\(resolvedVideoId)/maxresdefault.jpg") {
-                return [Thumbnail(url: url, quality: .maxres)]
+                return Thumbnail.fallbackChain(for: url)
             }
             // Fallback to proxy URL if video ID not available
             if let proxyURL = thumbnailUrl.flatMap({ URL(string: $0) }) {

Yattee/Services/Downloads/Download.swift

@@ -336,7 +336,7 @@ struct Download: Identifiable, Codable, Sendable, Equatable {
             publishedText: publishedText,
             viewCount: viewCount,
             likeCount: likeCount,
-            thumbnails: resolvedThumbnailURL.map { [Thumbnail(url: $0, quality: .medium)] } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: resolvedThumbnailURL),
             isLive: false,
             isUpcoming: false,
             scheduledStartTime: nil

Yattee/Services/Downloads/DownloadManager.swift

@@ -960,7 +960,7 @@ final class DownloadManager: NSObject {
             publishedText: download.publishedText,
             viewCount: download.viewCount,
             likeCount: download.likeCount,
-            thumbnails: download.thumbnailURL.map { [Thumbnail(url: $0, quality: .medium)] } ?? [],
+            thumbnails: Thumbnail.fallbackChain(for: download.thumbnailURL),
             isLive: false,
             isUpcoming: false,
             scheduledStartTime: nil