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/de.json

@@ -330,6 +330,7 @@
 	"comment_time_days": "vor {count} Tag(en)",
 	"comment_panel_title": "Kommentare",
 	"comment_panel_close": "Schließen",
+	"comp_dismiss": "Schließen",
 	"doc_panel_tab_metadata": "Metadaten",
 	"doc_panel_tab_transcription": "Transkription",
 	"doc_panel_tab_discussion": "Diskussion",
@@ -541,6 +542,7 @@
 	"training_start_btn": "Training starten",
 	"training_in_progress": "…",
 	"training_success": "Training wurde gestartet und abgeschlossen.",
+	"training_start_failed": "Training konnte nicht gestartet werden. Bitte versuche es erneut.",
 	"training_too_few_blocks": "Mindestens 5 geprüfte Blöcke erforderlich (aktuell: {available}).",
 	"training_service_down": "OCR-Dienst ist nicht erreichbar.",
 	"training_history_heading": "Verlauf",

frontend/messages/en.json

@@ -330,6 +330,7 @@
 	"comment_time_days": "{count} day(s) ago",
 	"comment_panel_title": "Comments",
 	"comment_panel_close": "Close",
+	"comp_dismiss": "Dismiss",
 	"doc_panel_tab_metadata": "Metadata",
 	"doc_panel_tab_transcription": "Transcription",
 	"doc_panel_tab_discussion": "Discussion",
@@ -541,6 +542,7 @@
 	"training_start_btn": "Start training",
 	"training_in_progress": "…",
 	"training_success": "Training started and completed.",
+	"training_start_failed": "Failed to start training. Please try again.",
 	"training_too_few_blocks": "At least 5 reviewed blocks required (currently: {available}).",
 	"training_service_down": "OCR service is unavailable.",
 	"training_history_heading": "History",

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",