feat(auth): enhance validation rules for username and password

- Updated username validation to allow only lowercase letters, numbers, and underscores.
- Strengthened password requirements to include at least 8 characters, one uppercase letter, one lowercase letter, one number, and one special character.
- Adjusted frontend forms and backend DTOs to reflect new validation rules.

backend/src/auth/auth.service.spec.ts

@@ -148,7 +148,7 @@ describe("AuthService", () => {
 			const dto = {
 				username: "test",
 				email: "test@example.com",
-				password: "password",
+				password: "Password1!",
 			};
 			mockHashingService.hashPassword.mockResolvedValue("hashed-password");
 			mockHashingService.hashEmail.mockResolvedValue("hashed-email");
@@ -165,7 +165,7 @@ describe("AuthService", () => {
 
 	describe("login", () => {
 		it("should login a user", async () => {
-			const dto = { email: "test@example.com", password: "password" };
+			const dto = { email: "test@example.com", password: "Password1!" };
 			const user = {
 				uuid: "user-id",
 				username: "test",