purple: fix dark mode issues

Some views appeared broken when iOS was in dark mode, with a white
background. This commit fixes that and makes sure the background is
always dark.

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Purple/DamusPurpleTranslationSetupView.swift

@@ -151,11 +151,10 @@ struct DamusPurpleTranslationSetupView: View {
         }
         .background(content: {
             ZStack {
-                Rectangle()
-                    .background(.black)
                 Image("purple-blue-gradient-1")
                     .offset(CGSize(width: 300.0, height: -0.0))
                     .opacity(start ? 1.0 : 0.2)
+                    .background(.black)
                 Image("stars-bg")
                     .resizable(resizingMode: .stretch)
                     .frame(width: 500, height: 500)

damus/Views/Purple/DamusPurpleVerifyNpubView.swift

@@ -15,6 +15,7 @@ struct DamusPurpleVerifyNpubView: View {
     var body: some View {
         ZStack {
             Rectangle()
+                .foregroundStyle(Color.black)
                 .background(Color.black)
             
             VStack {

damus/Views/Purple/DamusPurpleWelcomeView.swift

@@ -96,11 +96,10 @@ struct DamusPurpleWelcomeView: View {
         }
         .background(content: {
             ZStack {
-                Rectangle()
-                    .background(.black)
                 Image("purple-blue-gradient-1")
                     .offset(CGSize(width: 300.0, height: -0.0))
                     .opacity(start ? 1.0 : 0.2)
+                    .background(.black)
                 Image("stars-bg")
                     .resizable(resizingMode: .stretch)
                     .frame(width: 500, height: 500)