Add demo instance, remove public manifest

Shared/Views/ShareButton.swift

@@ -15,7 +15,9 @@ struct ShareButton: View {
         Menu {
             instanceActions
             Divider()
-            youtubeActions
+            if !accounts.isDemo {
+                youtubeActions
+            }
         } label: {
             Label("Share...", systemImage: "square.and.arrow.up")
         }

Shared/Views/WelcomeScreen.swift

@@ -67,7 +67,7 @@ struct WelcomeScreen: View {
                 .foregroundColor(.secondary)
         }
         .onAppear {
-            resource.load().onSuccess { response in
+            resource?.load().onSuccess { response in
                 if let instances: [ManifestedInstance] = response.typedContent() {
                     store = instances
                 }
@@ -85,7 +85,7 @@ struct WelcomeScreen: View {
         store.first { $0.country == country }?.flag
     }
 
-    var resource: Resource {
+    var resource: Resource? {
         InstancesManifest.shared.instancesList
     }
 }