Fix collapsed AddSource/EditSource sheets on macOS

Add macOS-only minimum frame to NavigationStack so the sheet has
intrinsic size. Matches the pattern already used in
PeerTubeInstancesExploreView.

Yattee/Views/Settings/AddSourceView.swift

@@ -88,6 +88,9 @@ struct AddSourceView: View {
                     )
                 }
         }
+        #if os(macOS)
+        .frame(minWidth: 500, minHeight: 450)
+        #endif
         .sheet(isPresented: $showingNetworkDiscovery) {
             NetworkShareDiscoverySheet(filterType: selectedShareType) { share in
                 handleSelectedShare(share)

Yattee/Views/Settings/EditSourceView.swift

@@ -101,6 +101,9 @@ private struct EditRemoteServerContent: View {
                 }
                 .accessibilityIdentifier("editSource.view")
         }
+        #if os(macOS)
+        .frame(minWidth: 500, minHeight: 450)
+        #endif
         #endif
     }