feat(bulk-upload): guard discard-all with confirm dialog

Uses getConfirmService() (optional — null fallback when context is absent so
unit tests that don't exercise the discard path need no CONFIRM_KEY context)
and the new bulk_discard_confirm i18n key.

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

frontend/messages/de.json

@@ -858,6 +858,7 @@
 	"bulk_save_cta_one": "Speichern →",
 	"bulk_save_cta": "{count} speichern →",
 	"bulk_discard_all": "Alle verwerfen",
+	"bulk_discard_confirm": "Alle Dateien und eingegebenen Daten verwerfen? Diese Aktion kann nicht rückgängig gemacht werden.",
 	"bulk_add_more": "Weitere hinzufügen",
 	"bulk_scope_per_file_label": "Nur diese Datei",
 	"bulk_scope_shared_label": "Gilt für alle {count}",

frontend/messages/en.json

@@ -858,6 +858,7 @@
 	"bulk_save_cta_one": "Save →",
 	"bulk_save_cta": "Save {count} →",
 	"bulk_discard_all": "Discard all",
+	"bulk_discard_confirm": "Discard all files and entered data? This action cannot be undone.",
 	"bulk_add_more": "Add more",
 	"bulk_scope_per_file_label": "This file only",
 	"bulk_scope_shared_label": "Applies to all {count}",

frontend/messages/es.json

@@ -858,6 +858,7 @@
 	"bulk_save_cta_one": "Guardar →",
 	"bulk_save_cta": "Guardar {count} →",
 	"bulk_discard_all": "Descartar todo",
+	"bulk_discard_confirm": "¿Descartar todos los archivos y datos introducidos? Esta acción no se puede deshacer.",
 	"bulk_add_more": "Añadir más",
 	"bulk_scope_per_file_label": "Solo este archivo",
 	"bulk_scope_shared_label": "Para todos los {count}",