Fix collapsed sheets on macOS across the app

Add macOS-only minimum frame sizing to sheets that wrap a
NavigationStack/Form without intrinsic size, so they render properly
instead of collapsing to just the toolbar. Affects Customize Home,
subscription/channel view options, playlist create/edit, search
filters, media browser options, instance picker, log filters, preset
editor, and legacy data import result.

Yattee/Views/Playlist/PlaylistFormSheet.swift

@@ -89,6 +89,9 @@ struct PlaylistFormSheet: View {
         #if os(iOS)
         .presentationDetents([.medium])
         #endif
+        #if os(macOS)
+        .frame(minWidth: 500, minHeight: 400)
+        #endif
     }
 
     // MARK: - Form Content