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 }
-            .first?
-            .thumbnailURL
+        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