Optimize SwiftUI performance throughout the app

This commit addresses multiple SwiftUI performance bottlenecks identified
through code analysis, focusing on view rendering efficiency, list
performance, and memory usage optimization.

Key improvements:

- HomeView: Optimize async task management using structured concurrency
  with async let to handle multiple Defaults updates in a single task

- VideoCell: Remove GeometryReader from VideoCellThumbnail to eliminate
  layout thrashing; change @ObservedObject to computed property for shared
  ThumbnailsModel

- ThumbnailView: Cache URL extension computation in init() instead of
  recalculating on every body evaluation

- FavoriteItemView: Replace filter().prefix() with early-exit loop and
  capacity reservation for significant performance gain on large lists

- ContentItemView: Optimize FetchRequest creation with direct predicate
  construction only for video items, empty predicate for others

- VideoPlayerView: Fix playerSize didSet trigger by moving
  updateSidebarQueue() calls to explicit onChange/onAppear handlers

- FeedView: Replace .unique() with Set-based deduplication for O(n)
  performance and reduced allocations

- VerticalCells: Remove expensive sorting on every redraw; items should
  be pre-sorted from source

These optimizations follow SwiftUI best practices by minimizing expensive
computations in view bodies, caching computed values, using efficient data
structures, and avoiding unnecessary redraws and layout passes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Shared/Subscriptions/FeedView.swift

@@ -15,16 +15,27 @@ struct FeedView: View {
     #endif
 
     var videos: [ContentItem] {
+        let feedVideos = feed.videos
         guard let selectedChannel else {
-            return ContentItem.array(of: feed.videos)
+            return ContentItem.array(of: feedVideos)
         }
-        return ContentItem.array(of: feed.videos.filter {
-            $0.channel.id == selectedChannel.id
-        })
+        return ContentItem.array(of: feedVideos.filter { $0.channel.id == selectedChannel.id })
     }
 
     var channels: [Channel] {
-        feed.videos.map(\.channel).unique()
+        // Optimize by using a Set for uniqueness instead of calling .unique()
+        var seenIds = Set<String>()
+        var uniqueChannels = [Channel]()
+        uniqueChannels.reserveCapacity(feed.videos.count / 10) // Estimate
+
+        for video in feed.videos {
+            let channelId = video.channel.id
+            if !seenIds.contains(channelId) {
+                seenIds.insert(channelId)
+                uniqueChannels.append(video.channel)
+            }
+        }
+        return uniqueChannels
     }
 
     @State private var selectedChannel: Channel?