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/Controllers/Auth/AppUserController.cs

@@ -0,0 +1,18 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+
+namespace API.Controllers.Auth
+{
+    [ApiController]
+    [Route("auth/user")]
+    public class AppUserController : ControllerBase
+    {
+        [HttpGet]
+        [Authorize]
+        public async Task<IActionResult> GetAppUsers()
+        {
+            return Ok();
+        }
+    }
+}