fix(import): surface S3 failures + already-exists in skippedFiles, a11y + max-height

- Change importSingleDocument return type from boolean to Optional<String>
  so callers in processRows receive the skip reason on every non-success path.
  S3 upload failures now surface as "S3_UPLOAD_FAILED" and already-imported
  documents as "ALREADY_EXISTS" in the skippedFiles list shown in the admin UI.
- Add two new tests: runImportAsync_addsS3UploadFailed_toSkippedFiles and
  runImportAsync_addsAlreadyExists_toSkippedFiles; update
  importSingleDocument_skips_whenDocumentAlreadyUploadedNotPlaceholder and
  the S3-failure test to assert on the Optional return value.
- Add i18n keys for S3_UPLOAD_FAILED and ALREADY_EXISTS in de/en/es messages.
- Svelte ImportStatusCard: add aria-hidden="true" to SVG chevron, wrap
  conditional warning section in aria-live="polite" div, add max-h-64
  overflow-y-auto to skipped-files <ul> to cap height on large batches.

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

frontend/messages/de.json

@@ -353,6 +353,8 @@
 	"admin_system_import_skipped_label": "übersprungen",
 	"import_reason_invalid_pdf_signature": "Keine gültige PDF-Signatur",
 	"import_reason_file_read_error": "Fehler beim Lesen der Datei",
+	"import_reason_s3_upload_failed": "Upload-Fehler (S3)",
+	"import_reason_already_exists": "Bereits importiert",
 	"admin_system_import_status_failed": "Import fehlgeschlagen",
 	"admin_system_import_failed_no_spreadsheet": "Keine Tabellendatei gefunden.",
 	"admin_system_import_failed_internal": "Interner Fehler beim Import.",

frontend/messages/en.json

@@ -353,6 +353,8 @@
 	"admin_system_import_skipped_label": "skipped",
 	"import_reason_invalid_pdf_signature": "Invalid PDF signature",
 	"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_internal": "Import failed due to an internal error.",

frontend/messages/es.json

@@ -353,6 +353,8 @@
 	"admin_system_import_skipped_label": "omitidos",
 	"import_reason_invalid_pdf_signature": "Firma PDF no válida",
 	"import_reason_file_read_error": "Error al leer el archivo",
+	"import_reason_s3_upload_failed": "Error de carga (S3)",
+	"import_reason_already_exists": "Ya importado",
 	"admin_system_import_status_failed": "Importación fallida",
 	"admin_system_import_failed_no_spreadsheet": "No se encontró ninguna hoja de cálculo.",
 	"admin_system_import_failed_internal": "Error interno durante la importación.",