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/layout.server.spec.ts

@@ -16,12 +16,12 @@ beforeEach(() => vi.clearAllMocks());
 describe('admin/users layout load', () => {
 	it('returns the users list', async () => {
 		mockApi([
-			{ id: 'u1', username: 'alice' },
-			{ id: 'u2', username: 'bob' }
+			{ id: 'u1', email: 'alice@example.com' },
+			{ id: 'u2', email: 'bob@example.com' }
 		]);
 		const result = await load({ fetch: vi.fn() as unknown as typeof fetch });
 		expect(result.users).toHaveLength(2);
-		expect(result.users[0].username).toBe('alice');
+		expect(result.users[0].email).toBe('alice@example.com');
 	});
 
 	it('returns an empty array when the API returns nothing', async () => {