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/+error.svelte

@@ -1,9 +1,10 @@
 <script lang="ts">
 import { page } from '$app/state';
+import { m } from '$lib/paraglide/messages.js';
 </script>
 
 <svelte:head>
-	<title>Fehler – Familienarchiv</title>
+	<title>{m.page_title_error()}</title>
 </svelte:head>
 
 <div class="px-4 py-12 text-center font-sans">

frontend/src/routes/+page.svelte

@@ -68,7 +68,7 @@ $effect(() => {
 </script>
 
 <svelte:head>
-	<title>Archiv</title>
+	<title>{m.page_title_home()}</title>
 </svelte:head>
 
 <main class="mx-auto max-w-7xl px-4 py-8 font-sans sm:px-6 lg:px-8">

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">

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">

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

@@ -24,7 +24,7 @@ function handleSearch() {
 </script>
 
 <svelte:head>
-	<title>Personen</title>
+	<title>{m.page_title_persons()}</title>
 </svelte:head>
 
 <div class="mx-auto max-w-7xl px-4 py-12 sm:px-6 lg:px-8">