refactor: introduce PurpleBackdrop

We mainly want to use this to improve previews for now, but might be
useful in the future.

Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Purple/DamusPurpleView.swift

@@ -44,14 +44,7 @@ struct DamusPurpleView: View, DamusPurpleStoreKitManagerDelegate {
     
     var body: some View {
         NavigationView {
-            ZStack {
-                Color.black
-                    .edgesIgnoringSafeArea(.all)
-
-                Image("purple-blue-gradient-1")
-                    .resizable()
-                    .edgesIgnoringSafeArea(.all)
-
+            PurpleBackdrop {
                 ScrollView {
                     MainContent
                         .padding(.top, 75)

damus/Views/Purple/Detail/PurpleBackdrop.swift

@@ -0,0 +1,30 @@
+//
+//  PurpleBackdrop.swift
+//  damus
+//
+//  Created by William Casarin on 2024-03-04.
+//
+
+import SwiftUI
+
+struct PurpleBackdrop<T: View>: View {
+    @ViewBuilder let content: () -> T
+    var body: some View {
+        ZStack {
+            Color.black
+                .edgesIgnoringSafeArea(.all)
+
+            Image("purple-blue-gradient-1")
+                .resizable()
+                .edgesIgnoringSafeArea(.all)
+
+            content()
+        }
+    }
+}
+
+#Preview {
+    PurpleBackdrop {
+        Text("Hello, World")
+    }
+}