Fix presentationCornerRadius iOS 16.0 availability

Wrap in a ViewModifier that guards with #available(iOS 16.4, *),
keeping the rounded sheet on 16.4+ while compiling clean on 16.0+.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Features/Login/LoginView.swift

@@ -97,7 +97,7 @@ struct LoginView: View {
                     navigateToDashboard = true
                 }
                 .presentationDetents([.height(200)])
-                .presentationCornerRadius(20)
+                .modifier(SheetCornerRadius(20))
             }
             .onAppear {
                 vm.onAuthenticated = { navigateToDashboard = true }
@@ -158,3 +158,15 @@ struct MoreOptionsSheet: View {
         .buttonStyle(ScaleButtonStyle())
     }
 }
+
+private struct SheetCornerRadius: ViewModifier {
+    let radius: CGFloat
+    init(_ radius: CGFloat) { self.radius = radius }
+    func body(content: Content) -> some View {
+        if #available(iOS 16.4, *) {
+            content.presentationCornerRadius(radius)
+        } else {
+            content
+        }
+    }
+}