refactor: notifications and settings view models; enhance login and registration UI

Mayday/Views/UIKit/AppBackground.swift

@@ -0,0 +1,20 @@
+import SwiftUI
+
+struct AppBackgroundModifier: ViewModifier {
+    func body(content: Content) -> some View {
+        content.background(
+            LinearGradient(
+                colors: [Color(.systemGroupedBackground), Color.red.opacity(0.08)],
+                startPoint: .top,
+                endPoint: .bottom
+            )
+            .ignoresSafeArea()
+        )
+    }
+}
+
+extension View {
+    func appBackground() -> some View {
+        modifier(AppBackgroundModifier())
+    }
+}