Adds a statusCode field (IMPORT_IDLE / IMPORT_RUNNING / IMPORT_DONE / IMPORT_FAILED_NO_SPREADSHEET / IMPORT_FAILED_INTERNAL) to ImportStatus. The frontend will map these codes to localized strings via Paraglide instead of rendering the backend's German message verbatim. NoSpreadsheetException distinguishes a missing spreadsheet from other I/O failures so the frontend can show a specific error without raw text. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>