Add change-password API and dynamic 404 redirect

Introduce ChangePasswordRequest DTO and a new ChangePassword endpoint in AuthController that validates input, changes the user's password via UserManager, updates the security stamp, signs out the user to invalidate sessions, and returns localized messages. Add a simple authorized AppUserController stub (GET /auth/user). Update the 404 view to resolve auth status via fetchCurrentUser, show a dynamic CTA/icon (Dashboard vs Home), auto-redirect after a short delay with proper timer cleanup, and adjust navigation behavior. Update codexInfo.md to document the 404 behavior change.

API/Contracts/Auth/ChangePasswordRequest.cs

@@ -0,0 +1,14 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace API.Contracts.Auth
+{
+    public class ChangePasswordRequest
+    {
+        [Required]
+        public string OldPassword { get; set; } = string.Empty;
+        [Required]
+        public string NewPassword { get; set; } = string.Empty;
+        [Required]
+        public string NewPasswordConfirm { get; set; } = string.Empty;
+    }
+}