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/users/[id]/+page.svelte

@@ -6,7 +6,7 @@ let { data } = $props();
 const fullName = $derived.by(() => {
 	const first = data.profileUser.firstName;
 	const last = data.profileUser.lastName;
-	return first || last ? [first, last].filter(Boolean).join(' ') : data.profileUser.username;
+	return first || last ? [first, last].filter(Boolean).join(' ') : data.profileUser.email;
 });
 
 const initials = $derived.by(() => {
@@ -70,12 +70,9 @@ const initials = $derived.by(() => {
 				{/if}
 			</div>
 
-			<!-- Name and username -->
+			<!-- Name -->
 			<div class="mb-5 text-center">
 				<h2 class="font-serif text-xl font-bold text-ink">{fullName}</h2>
-				{#if data.profileUser.firstName || data.profileUser.lastName}
-					<p class="mt-0.5 font-sans text-sm text-ink-3">@{data.profileUser.username}</p>
-				{/if}
 			</div>
 
 			<!-- Field rows -->