fix(frontend): show error on training start failure, add aria-live and dismiss to success message

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/es.json

@@ -330,6 +330,7 @@
 	"comment_time_days": "hace {count} día(s)",
 	"comment_panel_title": "Comentarios",
 	"comment_panel_close": "Cerrar",
+	"comp_dismiss": "Cerrar",
 	"doc_panel_tab_metadata": "Metadatos",
 	"doc_panel_tab_transcription": "Transcripción",
 	"doc_panel_tab_discussion": "Discusión",
@@ -541,6 +542,7 @@
 	"training_start_btn": "Iniciar entrenamiento",
 	"training_in_progress": "…",
 	"training_success": "Entrenamiento iniciado y completado.",
+	"training_start_failed": "No se pudo iniciar el entrenamiento. Por favor, inténtalo de nuevo.",
 	"training_too_few_blocks": "Se requieren al menos 5 bloques revisados (actualmente: {available}).",
 	"training_service_down": "El servicio OCR no está disponible.",
 	"training_history_heading": "Historial",