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/Home/HomeView.swift

@@ -118,6 +118,9 @@ struct HomeView: View {
                         }
                     }
             }
+            #if os(macOS)
+            .frame(minWidth: 500, minHeight: 450)
+            #endif
         }
         #endif
         .onAppear {