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/Models/Navigation/SidebarItem.swift

@@ -215,22 +215,6 @@ enum SidebarItem: Hashable, Identifiable {
 
     // MARK: - Item Categories
 
-    /// Whether this is a fixed navigation item (always visible).
-    var isFixedNavigation: Bool {
-        switch self {
-        case .home, .search, .sources, .settings, .nowPlaying, .openURL, .remoteControl:
-            return true
-        default:
-            return false
-        }
-    }
-
-    /// Whether this is a dynamic channel item.
-    var isChannel: Bool {
-        if case .channel = self { return true }
-        return false
-    }
-
     /// Whether this is a dynamic playlist item.
     var isPlaylist: Bool {
         switch self {