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.server.ts

@@ -5,14 +5,14 @@ import { getErrorMessage } from '$lib/errors';
 export const actions = {
 	login: async ({ request, cookies, fetch }) => {
 		const data = await request.formData();
-		const username = data.get('username') as string;
+		const email = data.get('email') as string;
 		const password = data.get('password') as string;
 
-		if (!username || !password) {
-			return fail(400, { error: 'Bitte Benutzername und Passwort eingeben.' });
+		if (!email || !password) {
+			return fail(400, { error: getErrorMessage('MISSING_CREDENTIALS') });
 		}
 
-		const credentials = btoa(`${username}:${password}`);
+		const credentials = btoa(`${email}:${password}`);
 		const authHeader = `Basic ${credentials}`;
 
 		// Raw fetch is intentional here: we need to pass an explicit Authorization