Add a view with instructions for enabling backups in Element Classic. (#5359)

No presentation logic yet.

PreviewTests/Sources/GeneratedPreviewTests.swift

@@ -83,6 +83,14 @@ extension PreviewTests {
         }
     }
 
+    @Test
+    func authenticationClassicAppBackupInstructionsView() async throws {
+        AppSettings.resetAllSettings() // Ensure this test's previews start with fresh settings.
+        for (index, preview) in AuthenticationClassicAppBackupInstructionsView_Previews._allPreviews.enumerated() {
+            try await assertSnapshots(matching: preview, step: index)
+        }
+    }
+
     @Test
     func authenticationStartScreen() async throws {
         AppSettings.resetAllSettings() // Ensure this test's previews start with fresh settings.