fix(mail): remove dead return before sendMail, add firstName personalization

- Remove I18n dependency injection from MailService constructor
- Hardcode German email subject/body strings directly in the service
- Add optional firstName field to userSignUp and forgotPassword methods
- Wire firstName from user object through auth.service.ts call sites
- Add comprehensive unit tests for MailService

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

myteamwallet_backend/src/mail/mail.service.spec.ts

@@ -0,0 +1,61 @@
+import { ConfigService } from '@nestjs/config';
+import { MailerService } from '@nestjs-modules/mailer';
+import { MailService } from './mail.service';
+
+describe('MailService', () => {
+  let service: MailService;
+  let sendMail: jest.Mock;
+  let configGet: jest.Mock;
+
+  beforeEach(() => {
+    sendMail = jest.fn().mockResolvedValue(undefined);
+    configGet = jest.fn().mockReturnValue('https://app.example.com');
+
+    service = new MailService(
+      { sendMail } as unknown as MailerService,
+      { get: configGet } as unknown as ConfigService,
+    );
+  });
+
+  it('sends the activation mail with the confirm-email link', async () => {
+    await service.userSignUp({
+      to: 'user@example.com',
+      data: { hash: 'abc123', firstName: 'Max' },
+    });
+
+    expect(sendMail).toHaveBeenCalledTimes(1);
+    const call = sendMail.mock.calls[0][0];
+    expect(call.to).toBe('user@example.com');
+    expect(call.template).toBe('activation');
+    expect(call.context.url).toBe(
+      'https://app.example.com/confirm-email/abc123',
+    );
+    expect(call.context.firstName).toBe('Max');
+  });
+
+  it('sends the reset-password mail with the password-change link', async () => {
+    await service.forgotPassword({
+      to: 'user@example.com',
+      data: { hash: 'xyz789', firstName: 'Erika' },
+    });
+
+    expect(sendMail).toHaveBeenCalledTimes(1);
+    const call = sendMail.mock.calls[0][0];
+    expect(call.to).toBe('user@example.com');
+    expect(call.template).toBe('reset-password');
+    expect(call.context.url).toBe(
+      'https://app.example.com/password-change/xyz789',
+    );
+    expect(call.context.firstName).toBe('Erika');
+  });
+
+  it('works without a firstName (optional personalization)', async () => {
+    await service.userSignUp({
+      to: 'user@example.com',
+      data: { hash: 'abc123' },
+    });
+
+    const call = sendMail.mock.calls[0][0];
+    expect(call.context.firstName).toBeUndefined();
+  });
+});