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/admin/users/new/page.svelte.spec.ts

@@ -22,9 +22,10 @@ describe('Admin new user page – rendering', () => {
 		await expect.element(page.getByText(/Neuen Benutzer anlegen/i)).toBeInTheDocument();
 	});
 
-	it('renders the login input', async () => {
+	it('renders the email input', async () => {
 		render(Page, { data: baseData, form: null });
-		await expect.element(page.getByRole('textbox', { name: /Login/i })).toBeInTheDocument();
+		const input = document.querySelector<HTMLInputElement>('input[name="email"]');
+		expect(input).not.toBeNull();
 	});
 
 	it('renders group checkboxes for each available group', async () => {