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/admin/auth.ts

@@ -0,0 +1,33 @@
+import { NextApiHandler } from "next";
+import { signAdminToken, checkPassword, COOKIE_NAME, getTokenFromRequest, verifyAdminToken } from "@utils/admin-auth";
+
+const handler: NextApiHandler = async (req, res) => {
+    if (req.method === "POST") {
+        const { password } = req.body ?? {};
+        if (!password || typeof password !== "string") {
+            return res.status(400).json({ error: "Password required" });
+        }
+        if (!checkPassword(password)) {
+            return res.status(401).json({ error: "Invalid password" });
+        }
+        const token = await signAdminToken();
+        res.setHeader("Set-Cookie", `${COOKIE_NAME}=${token}; HttpOnly; Path=/; SameSite=Lax; Max-Age=28800`);
+        return res.status(200).json({ ok: true });
+    }
+
+    if (req.method === "DELETE") {
+        res.setHeader("Set-Cookie", `${COOKIE_NAME}=; HttpOnly; Path=/; Max-Age=0`);
+        return res.status(200).json({ ok: true });
+    }
+
+    if (req.method === "GET") {
+        const token = getTokenFromRequest(req);
+        const valid = token ? await verifyAdminToken(token) : false;
+        return res.status(200).json({ authenticated: valid });
+    }
+
+    res.setHeader("Allow", ["GET", "POST", "DELETE"]);
+    return res.status(405).json({ error: "Method Not Allowed" });
+};
+
+export default handler;