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/login/+page.svelte

@@ -10,7 +10,7 @@ const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
 <svelte:head>
-	<title>Anmelden</title>
+	<title>{m.page_title_login()}</title>
 </svelte:head>
 
 <div class="relative flex min-h-screen flex-col bg-canvas">