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/[id]/page.svelte.spec.ts

@@ -16,7 +16,6 @@ const groups = [
 
 const makeUser = (overrides = {}) => ({
 	id: 'u1',
-	username: 'max',
 	firstName: 'Max',
 	lastName: 'Mustermann',
 	email: 'max@example.com',
@@ -52,9 +51,11 @@ afterEach(cleanup);
 // ─── Rendering ────────────────────────────────────────────────────────────────
 
 describe('Admin edit user page – rendering', () => {
-	it('renders the heading with username', async () => {
+	it('renders the heading with email', async () => {
 		renderPage({ data: baseData, form: null });
-		await expect.element(page.getByText(/Benutzer bearbeiten: max/i)).toBeInTheDocument();
+		await expect
+			.element(page.getByText(/Benutzer bearbeiten: max@example.com/i))
+			.toBeInTheDocument();
 	});
 
 	it('pre-fills first name from editUser data', async () => {