Fix feed channel filter avatars showing placeholders instead of images

The filter strip was passing the Invidious instance URL as serverURL to
AvatarURLBuilder, which built a Yattee Server-style /avatar/ path that
doesn't exist on Invidious. Now passes the actual Yattee Server URL
(matching SubscriptionsView pattern) and enriches channels from
CachedChannelData as a fallback when the API doesn't return thumbnails.

Yattee/Models/Channel.swift

@@ -94,6 +94,27 @@ struct ChannelID: Codable, Hashable, Sendable {
     }
 }
 
+extension Channel {
+    /// Returns a copy with `thumbnailURL` filled from cached channel data if currently nil.
+    @MainActor
+    func enrichedThumbnail(using dataManager: DataManager) -> Channel {
+        guard thumbnailURL == nil else { return self }
+        guard let cached = CachedChannelData.load(for: id.channelID, using: dataManager) else {
+            return self
+        }
+        return Channel(
+            id: id,
+            name: name,
+            description: description,
+            subscriberCount: subscriberCount ?? cached.subscriberCount,
+            videoCount: videoCount,
+            thumbnailURL: cached.thumbnailURL,
+            bannerURL: bannerURL ?? cached.bannerURL,
+            isVerified: isVerified
+        )
+    }
+}
+
 extension ChannelID: Identifiable {
     var id: String {
         switch source {

Yattee/Views/Instances/InstanceBrowseView.swift

@@ -67,12 +67,24 @@ struct InstanceBrowseView: View {
         appEnvironment?.settingsManager.listStyle ?? .inset
     }
 
-    /// Auth header for Yattee Server instances
+    /// The first enabled Yattee Server instance (for avatar URLs).
+    private var yatteeServer: Instance? {
+        appEnvironment?.instancesManager.enabledYatteeServerInstances.first
+    }
+    private var yatteeServerURL: URL? { yatteeServer?.url }
+
+    /// Auth header for Yattee Server instances (when browsing a Yattee Server directly)
     private var yatteeServerAuthHeader: String? {
         guard instance.type == .yatteeServer else { return nil }
         return appEnvironment?.yatteeServerCredentialsManager.basicAuthHeader(for: instance)
     }
 
+    /// Auth header for avatar loading (uses Yattee Server for YouTube channel avatars)
+    private var avatarAuthHeader: String? {
+        guard let server = yatteeServer else { return nil }
+        return appEnvironment?.yatteeServerCredentialsManager.basicAuthHeader(for: server)
+    }
+
     enum BrowseTab: String, CaseIterable, Identifiable {
         case popular
         case trending
@@ -425,7 +437,7 @@ struct InstanceBrowseView: View {
                         channelID: subscription.id.channelID,
                         name: subscription.name,
                         avatarURL: subscription.thumbnailURL,
-                        serverURL: instance.url,
+                        serverURL: yatteeServerURL,
                         isSelected: selectedFeedChannelID == subscription.id.channelID,
                         avatarSize: 44,
                         onTap: {
@@ -441,7 +453,7 @@ struct InstanceBrowseView: View {
                             )
                         },
                         onUnsubscribe: nil,
-                        authHeader: yatteeServerAuthHeader
+                        authHeader: avatarAuthHeader
                     )
                 }
             }
@@ -929,7 +941,7 @@ struct InstanceBrowseView: View {
                     return
                 }
 
-                feedSubscriptions = subscriptionChannels
+                feedSubscriptions = subscriptionChannels.map { $0.enrichedThumbnail(using: appEnvironment.dataManager) }
                 feedVideos = videos
                 prefetchBranding(for: videos)
             case .playlists: