Remove dead code found by Periphery on all platforms

Delete declarations that Periphery reports unused on every platform that
compiles them (iOS, macOS, tvOS), including nine views used only by their
own previews. Platform-gated code still used on another platform, device-
only MPV render paths, migration models, CloudKit record names, App
Intents metadata and unused parameters are kept.

Yattee/Views/Channel/ChannelView.swift

@@ -307,11 +307,6 @@ struct ChannelView: View {
         avatarSize - (avatarSize - collapsedAvatarSize) * collapseProgress
     }
 
-    /// Opacity for expanded content (fades out as we scroll)
-    private var expandedContentOpacity: CGFloat {
-        1 - min(collapseProgress * 2, 1)
-    }
-
     /// Opacity for collapsed title (fades in as we scroll)
     private var collapsedTitleOpacity: CGFloat {
         max((collapseProgress - 0.5) * 2, 0)
@@ -785,11 +780,6 @@ struct ChannelView: View {
         .animation(.easeInOut(duration: 0.25), value: isDescriptionScrollLocked)
     }
 
-    @ViewBuilder
-    private func tvOSAvatar(for channel: Channel) -> some View {
-        tvOSAvatar(name: channel.name, thumbnailURL: channel.thumbnailURL)
-    }
-
     @ViewBuilder
     private func tvOSAvatar(name: String, thumbnailURL: URL?) -> some View {
         LazyImage(url: thumbnailURL) { state in
@@ -2246,13 +2236,6 @@ struct ChannelView: View {
         isSearchLoading = false
     }
 
-    private func clearSearch() {
-        isSearchActive = false
-        hasSearched = false
-        searchResults = .empty
-        searchText = ""
-    }
-
     // MARK: - Error View
 
     private func errorView(_ error: String) -> some View {
@@ -2272,11 +2255,6 @@ struct ChannelView: View {
 
     // MARK: - Helpers
 
-    private func formatSubscribers(_ count: Int) -> String {
-        let formatted = CountFormatter.compact(count)
-        return String(localized: "channel.subscriberCount \(formatted)")
-    }
-
     /// Returns the appropriate instance for the channel's content source
     private func instanceForSource() -> Instance? {
         appEnvironment?.instancesManager.instance(for: source)