feat(auth): migrate frontend from username to email-only authentication

- Login page: email input replaces username field (type=email, name=email)
- Login server action: reads email, uses i18n error for missing credentials
- AccountSection: email input (type=email) replaces username text field
- New user server action: sends email as required field, drops username
- UsersListPanel: displays and searches by email instead of username
- Admin edit user page: heading and delete confirm use email
- Profile page: fullName fallback uses email, drops @username display
- app.d.ts: email required on User, username removed
- Generated API types: AppUser.email required, username removed; CreateUserRequest.email required, username removed
- i18n: login_label_email, login_error_missing_credentials, admin_col_login updated (de/en/es)
- errors.ts: MISSING_CREDENTIALS → login_error_missing_credentials

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/login/page.svelte.spec.ts

@@ -21,10 +21,10 @@ describe('Login page – rendering', () => {
 		await expect.element(page.getByRole('button', { name: 'Anmelden' })).toBeInTheDocument();
 	});
 
-	it('renders the username input', async () => {
+	it('renders the email input', async () => {
 		render(LoginPage, {});
 		await tick();
-		const input = document.querySelector<HTMLInputElement>('input[name="username"]');
+		const input = document.querySelector<HTMLInputElement>('input[name="email"]');
 		expect(input).not.toBeNull();
 	});
 
@@ -35,10 +35,10 @@ describe('Login page – rendering', () => {
 		expect(input).not.toBeNull();
 	});
 
-	it('username field is required', async () => {
+	it('email field is required', async () => {
 		render(LoginPage, {});
 		await tick();
-		const input = document.querySelector<HTMLInputElement>('input[name="username"]');
+		const input = document.querySelector<HTMLInputElement>('input[name="email"]');
 		expect(input?.required).toBe(true);
 	});
 
@@ -49,6 +49,13 @@ describe('Login page – rendering', () => {
 		expect(input?.required).toBe(true);
 	});
 
+	it('email field has type="email"', async () => {
+		render(LoginPage, {});
+		await tick();
+		const input = document.querySelector<HTMLInputElement>('input[name="email"]');
+		expect(input?.type).toBe('email');
+	});
+
 	it('password field has type="password"', async () => {
 		render(LoginPage, {});
 		await tick();