i18n: translate page titles (home, persons, admin, login, error)

Replaces hardcoded German strings with Paraglide message keys
(page_title_home/persons/admin/login/error) across de/en/es.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -12,7 +12,7 @@ let activeTab = $state('users');
 </script>
 
 <svelte:head>
-	<title>Administration</title>
+	<title>{m.page_title_admin()}</title>
 </svelte:head>
 
 <div class="mx-auto max-w-7xl px-4 py-8 font-sans sm:px-6 lg:px-8">