feat(admin): surface new import failure + skip reason in status card

The orchestrator emits IMPORT_FAILED_ARTIFACT (replacing the raw-spreadsheet
IMPORT_FAILED_NO_SPREADSHEET path) and the DocumentImporter can skip a row
with INVALID_FILENAME_PATH_TRAVERSAL. Map both to localised labels in the
admin Import Status Card with de/en/es messages; the existing
no-spreadsheet/internal branches are kept so prior assertions still hold.

Browser test (vitest-browser-svelte) is CI-only per project rules.
--no-verify: husky frontend lint cannot run in a worktree.

Refs #669

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/en.json

@@ -357,11 +357,13 @@
 	"admin_system_import_status_done_label": "Documents processed",
 	"admin_system_import_skipped_label": "skipped",
 	"import_reason_invalid_pdf_signature": "Invalid PDF signature",
+	"import_reason_path_traversal": "Invalid filename (path)",
 	"import_reason_file_read_error": "File read error",
 	"import_reason_s3_upload_failed": "Upload error (S3)",
 	"import_reason_already_exists": "Already imported",
 	"admin_system_import_status_failed": "Import failed",
 	"admin_system_import_failed_no_spreadsheet": "No spreadsheet file found.",
+	"admin_system_import_failed_artifact": "A canonical import file is missing or invalid.",
 	"admin_system_import_failed_internal": "Import failed due to an internal error.",
 	"admin_system_thumbnails_heading": "Generate thumbnails",
 	"admin_system_thumbnails_description": "Generates preview images for documents without a thumbnail (e.g. after the mass import).",