Present instance login as full-screen cover on tvOS

The .sheet rendering on tvOS produced a tiny floating modal where the
"Sign In" title wrapped onto two lines and form fields overflowed. Use
.fullScreenCover on tvOS and wrap the login form in
TVSidebarDetailContainer so the title/icon sit in the standard 400pt
left sidebar. iOS and macOS keep the existing sheet presentation.

Yattee/Views/Settings/EditSourceView.swift

@@ -353,12 +353,21 @@ private struct EditRemoteServerContent: View {
             Button(String(localized: "common.cancel"), role: .cancel) {}
         }
         .presentationCompactAdaptation(.sheet)
+        #if os(tvOS)
+        .fullScreenCover(isPresented: $showLoginSheet) {
+            InstanceLoginView(instance: instance) { credential in
+                appEnvironment?.credentialsManager(for: instance)?.setCredential(credential, for: instance)
+                isLoggedIn = true
+            }
+        }
+        #else
         .sheet(isPresented: $showLoginSheet) {
             InstanceLoginView(instance: instance) { credential in
                 appEnvironment?.credentialsManager(for: instance)?.setCredential(credential, for: instance)
                 isLoggedIn = true
             }
         }
+        #endif
         .onAppear {
             isLoggedIn = appEnvironment?.credentialsManager(for: instance)?.isLoggedIn(for: instance) ?? false