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.svelte

@@ -32,16 +32,16 @@ let { form }: { form?: { error?: string; success?: boolean } } = $props();
 				<form method="POST" action="?/login" class="space-y-5">
 					<div>
 						<label
-							for="username"
+							for="email"
 							class="mb-1.5 block font-sans text-xs font-bold tracking-widest text-ink-2 uppercase"
-							>{m.login_label_username()}</label
+							>{m.login_label_email()}</label
 						>
 						<input
-							type="text"
-							name="username"
-							id="username"
+							type="email"
+							name="email"
+							id="email"
 							required
-							autocomplete="username"
+							autocomplete="email"
 							class="block w-full border border-line px-3 py-2.5 font-serif text-sm text-ink placeholder-ink-3 focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 						/>
 					</div>