feat: add admin panel, Replicate AI translation, and document translation

- Admin panel (/admin) with JWT auth: configure Replicate API token,
  JigsawStack API key, model version, enable/disable AI translation,
  change admin password. Settings persisted in data/settings.json.

- Replicate AI translation: POST /api/translate/replicate uses
  JigsawStack text-translate model via Replicate API. Main page
  switches to client-side AI translation when enabled.

- Document translation tab: supports PDF, DOCX, XLSX, XLS, CSV.
  Excel/Word formatting fully preserved (SheetJS + JSZip XML manipulation).
  PDF uses pdf-parse extraction + pdf-lib reconstruction.
  Column selector UI for tabular data (per-sheet, All/None toggles).

- Updated README with full implementation documentation.

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

components/index.tsx

@@ -7,3 +7,5 @@ export { default as ColorModeToggler } from "./ColorModeToggler";
 export { default as LangSelect } from "./LangSelect";
 export { default as TranslationArea } from "./TranslationArea";
 export { default as AutoTranslateButton } from "./AutoTranslateButton";
+export { default as DocumentTranslator } from "./DocumentTranslator";
+export { default as ColumnSelector } from "./ColumnSelector";