fix(import): address PR review concerns

- remove duplicate List import in AdminControllerTest
- derive skipped() from skippedFiles.size() — drop redundant int field
- use machine codes for SkippedFile.reason (INVALID_PDF_SIGNATURE, FILE_READ_ERROR)
- map reason codes to i18n strings in ImportStatusCard (de/en/es)
- replace raw amber Tailwind classes with warning semantic token
- fix <summary> accessibility: replace list-none with rotating chevron SVG
- replace <p> with <span> inside <summary> (phrasing content rule)
- extract setupOneValidOneFakeImport() helper — remove 3x copy-paste
- add lenient mock to short-file test for defensive coverage
- add IOException path test for isPdfMagicBytes

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

frontend/messages/en.json

@@ -353,6 +353,8 @@
 	"admin_system_import_status_done": "Import complete",
 	"admin_system_import_status_done_label": "Documents processed",
 	"admin_system_import_skipped_label": "skipped",
+	"import_reason_invalid_pdf_signature": "Invalid PDF signature",
+	"import_reason_file_read_error": "File read error",
 	"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.",