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>

next.config.js

@@ -5,6 +5,20 @@ module.exports = withPWA({
     pwa: {
         dest: "public"
     },
+    // Exclude server-only packages from client bundles
+    webpack: (config, { isServer }) => {
+        if (!isServer) {
+            config.resolve.fallback = {
+                ...config.resolve.fallback,
+                fs: false,
+                path: false,
+                crypto: false,
+                stream: false,
+                zlib: false
+            };
+        }
+        return config;
+    },
     async headers() {
         return [
             {