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/Header.tsx

@@ -3,6 +3,7 @@ import Head from "next/head";
 import NextLink from "next/link";
 import { Flex, HStack, IconButton, Link, useColorModeValue } from "@chakra-ui/react";
 import { FaGithub } from "react-icons/fa";
+import { FiSettings } from "react-icons/fi";
 import Image from "next/image";
 import { ColorModeToggler } from ".";
 
@@ -40,6 +41,15 @@ const Header: FC<Props> = (props) => (
                 <ColorModeToggler
                     variant={useColorModeValue("outline", "solid")}
                 />
+                <NextLink href="/admin" passHref={true}>
+                    <IconButton
+                        as={Link}
+                        aria-label="Admin settings"
+                        icon={<FiSettings />}
+                        colorScheme="lingva"
+                        variant={useColorModeValue("outline", "solid")}
+                    />
+                </NextLink>
                 <IconButton
                     as={Link}
                     href="https://github.com/thedaviddelta/lingva-translate"