fix(auth): use the real app icon on the login screen, not a placeholder

AuthHeaderView was still a generic gradient-circle + SF Symbol book icon
from before AppIcon.appiconset had real branded artwork — never swapped
out once the actual icon was added. Now renders Image("AppIcon") directly
so the login screen can't drift from whatever the Dock/Home Screen icon
actually is.

Outpost/Features/Auth/AuthHeaderView.swift

@@ -3,21 +3,15 @@ import SwiftUI
 struct AuthHeaderView: View {
     var body: some View {
         VStack(spacing: 12) {
-            ZStack {
-                Circle()
-                    .fill(
-                        LinearGradient(
-                            colors: [Color.accentColor, Color.accentColor.opacity(0.6)],
-                            startPoint: .topLeading,
-                            endPoint: .bottomTrailing
-                        )
-                    )
-                    .frame(width: 64, height: 64)
-                Image(systemName: "text.book.closed.fill")
-                    .font(.system(size: 26, weight: .semibold))
-                    .foregroundStyle(.white)
-            }
-            .shadow(color: Color.accentColor.opacity(0.35), radius: 12, y: 6)
+            // Renders whichever platform variant AppIcon.appiconset has for
+            // the current destination — same asset Xcode uses for the
+            // Dock/Home Screen icon, so this can't drift from it separately.
+            Image("AppIcon")
+                .resizable()
+                .scaledToFit()
+                .frame(width: 72, height: 72)
+                .clipShape(RoundedRectangle(cornerRadius: 72 * 0.2237, style: .continuous))
+                .shadow(color: .black.opacity(0.25), radius: 12, y: 6)
 
             VStack(spacing: 4) {
                 Text("Welcome to Outpost")