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/app.d.ts

@@ -5,11 +5,10 @@ declare global {
 		// Define the User structure matching your Java Entity
 		interface User {
 			id: string;
-			username: string;
 			firstName?: string;
 			lastName?: string;
 			birthDate?: string;
-			email?: string;
+			email: string;
 			contact?: string;
 			groups: {
 				id: string;