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/messages/de.json

@@ -307,5 +307,10 @@
 	"notification_prefs_no_email": "Bitte trage zuerst eine E-Mail-Adresse ein, um Benachrichtigungen zu erhalten.",
 	"notification_unread": "ungelesen",
 	"mention_btn_label": "Person erwähnen",
-	"mention_popup_empty": "Keine Nutzer gefunden"
+	"mention_popup_empty": "Keine Nutzer gefunden",
+	"page_title_home": "Archiv",
+	"page_title_persons": "Personen",
+	"page_title_admin": "Administration",
+	"page_title_login": "Anmelden",
+	"page_title_error": "Fehler – Familienarchiv"
 }

frontend/messages/en.json

@@ -307,5 +307,10 @@
 	"notification_prefs_no_email": "Please add an email address above to receive notifications.",
 	"notification_unread": "unread",
 	"mention_btn_label": "Mention person",
-	"mention_popup_empty": "No users found"
+	"mention_popup_empty": "No users found",
+	"page_title_home": "Archive",
+	"page_title_persons": "Persons",
+	"page_title_admin": "Administration",
+	"page_title_login": "Sign in",
+	"page_title_error": "Error – Family Archive"
 }

frontend/messages/es.json

@@ -307,5 +307,10 @@
 	"notification_prefs_no_email": "Por favor, añade una dirección de correo electrónico para recibir notificaciones.",
 	"notification_unread": "no leído",
 	"mention_btn_label": "Mencionar persona",
-	"mention_popup_empty": "No se encontraron usuarios"
+	"mention_popup_empty": "No se encontraron usuarios",
+	"page_title_home": "Archivo",
+	"page_title_persons": "Personas",
+	"page_title_admin": "Administración",
+	"page_title_login": "Iniciar sesión",
+	"page_title_error": "Error – Archivo familiar"
 }