fix(document): polish PDF error state — warning icon, 44px target, warmer copy

Address the remaining UI/UX polish: add a warning-triangle icon so the
failure is signalled by shape, not colour alone (WCAG 1.4.1); give the
recovery download link a full 44px tap/focus target (inline-flex
min-h-[44px]); and soften the message copy in de/en/es.

Addresses re-review: Leonie (colour-only, undersized link, copy warmth).

Refs #708

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

frontend/messages/de.json

@@ -126,7 +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_render_failed": "Dieser Scan ließ sich hier leider nicht anzeigen.",
 	"doc_no_scan": "Kein Scan vorhanden",
 	"persons_heading": "Personenverzeichnis",
 	"persons_subtitle": "Durchsuchen Sie den Index aller erfassten Personen im Familienarchiv.",

frontend/messages/en.json

@@ -126,7 +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_render_failed": "This scan couldn’t be displayed here.",
 	"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

@@ -126,7 +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_render_failed": "No se pudo mostrar este escaneo aquí.",
 	"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.",