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/AccountSection.svelte

@@ -11,9 +11,10 @@ import { m } from '$lib/paraglide/messages.js';
 		{m.admin_col_login()}
 	</span>
 	<input
-		type="text"
-		name="username"
+		type="email"
+		name="email"
 		required
+		autocomplete="email"
 		class="w-full rounded-sm border border-line px-3 py-2 font-serif text-sm focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 	/>
 </label>