i18n(document): add doc_render_failed message for blank-render fallback

Localized message shown when a PDF page cannot be rendered, so users
never see a blank canvas or a raw English pdf.js string. de/en/es.

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -126,6 +126,7 @@
 	"doc_label_summary": "Zusammenfassung",
 	"doc_loading": "Lade Dokument...",
 	"doc_download_link": "Direkter Download versuchen",
+	"doc_render_failed": "Dieser Scan konnte nicht angezeigt werden.",
 	"doc_no_scan": "Kein Scan vorhanden",
 	"persons_heading": "Personenverzeichnis",
 	"persons_subtitle": "Durchsuchen Sie den Index aller erfassten Personen im Familienarchiv.",

frontend/messages/en.json

@@ -126,6 +126,7 @@
 	"doc_label_summary": "Summary",
 	"doc_loading": "Loading document...",
 	"doc_download_link": "Try direct download",
+	"doc_render_failed": "This scan could not be displayed.",
 	"doc_no_scan": "No scan available",
 	"persons_heading": "Person directory",
 	"persons_subtitle": "Browse the index of all recorded persons in the family archive.",

frontend/messages/es.json

@@ -126,6 +126,7 @@
 	"doc_label_summary": "Resumen",
 	"doc_loading": "Cargando documento...",
 	"doc_download_link": "Intentar descarga directa",
+	"doc_render_failed": "No se pudo mostrar este escaneo.",
 	"doc_no_scan": "No hay escaneo disponible",
 	"persons_heading": "Directorio de personas",
 	"persons_subtitle": "Explore el índice de todas las personas registradas en el archivo familiar.",