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>

pages/api/translate/replicate.ts

@@ -0,0 +1,38 @@
+import { NextApiHandler } from "next";
+import NextCors from "nextjs-cors";
+import { replicateTranslate } from "@utils/replicate-translate";
+import { readSettings } from "@utils/settings-store";
+
+type Data = { translation: string } | { error: string };
+
+const handler: NextApiHandler<Data> = async (req, res) => {
+    await NextCors(req, res, { methods: ["POST"], origin: "*" });
+
+    if (req.method !== "POST") {
+        res.setHeader("Allow", ["POST"]);
+        return res.status(405).json({ error: "Method Not Allowed" });
+    }
+
+    const settings = readSettings();
+    if (!settings.replicateEnabled) {
+        return res.status(503).json({ error: "Replicate translation is not enabled" });
+    }
+
+    const { text, targetLanguage } = req.body ?? {};
+    if (!text || typeof text !== "string") {
+        return res.status(400).json({ error: "text is required" });
+    }
+    if (!targetLanguage || typeof targetLanguage !== "string") {
+        return res.status(400).json({ error: "targetLanguage is required" });
+    }
+
+    try {
+        const translation = await replicateTranslate(text, targetLanguage);
+        return res.status(200).json({ translation });
+    } catch (err) {
+        const msg = err instanceof Error ? err.message : "Translation failed";
+        return res.status(500).json({ error: msg });
+    }
+};
+
+export default handler;