ui: add DamusBackground helper

We will be using this in more places

damus/Components/Gradients/DamusBackground.swift

@@ -0,0 +1,30 @@
+//
+//  DamusBackground.swift
+//  damus
+//
+//  Created by William Casarin on 2023-07-12.
+//
+
+import Foundation
+import SwiftUI
+
+struct DamusBackground: View {
+    let maxHeight: CGFloat
+
+    init(maxHeight: CGFloat = 250.0) {
+        self.maxHeight = maxHeight
+    }
+
+    var body: some View {
+        Image("login-header")
+            .resizable()
+            .frame(maxWidth: .infinity, maxHeight: maxHeight, alignment: .center)
+            .ignoresSafeArea()
+    }
+}
+
+struct DamusBackground_Previews: PreviewProvider {
+    static var previews: some View {
+        DamusBackground()
+    }
+}