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>

utils/document-processors/pdf.ts

@@ -0,0 +1,115 @@
+import { PDFDocument, rgb, StandardFonts } from "pdf-lib";
+import { replicateTranslateBatch } from "../replicate-translate";
+
+type PdfParseResult = {
+    numpages: number;
+    text: string;
+    info: Record<string, unknown>;
+};
+
+async function parsePdf(buffer: Buffer): Promise<PdfParseResult> {
+    // Avoid Next.js issues with pdf-parse test file imports
+    // eslint-disable-next-line @typescript-eslint/no-var-requires
+    const pdfParse = require("pdf-parse/lib/pdf-parse.js");
+    return pdfParse(buffer);
+}
+
+function wrapText(text: string, maxCharsPerLine: number): string[] {
+    const words = text.split(/\s+/);
+    const lines: string[] = [];
+    let current = "";
+
+    for (const word of words) {
+        if ((current + " " + word).trim().length > maxCharsPerLine) {
+            if (current) lines.push(current);
+            current = word;
+        } else {
+            current = current ? current + " " + word : word;
+        }
+    }
+    if (current) lines.push(current);
+    return lines;
+}
+
+/**
+ * Translate a PDF buffer. Since PDFs don't support in-place text editing,
+ * this extracts text, translates it, and creates a new formatted PDF.
+ */
+export async function translatePdf(
+    buffer: Buffer,
+    targetLanguage: string,
+    sourceLanguage?: string
+): Promise<Buffer> {
+    const parsed = await parsePdf(buffer);
+    const rawText = parsed.text;
+
+    // Split into paragraphs (separated by double newlines or page breaks)
+    const paragraphs = rawText
+        .split(/\n{2,}|\f/)
+        .map(p => p.replace(/\n/g, " ").trim())
+        .filter(p => p.length > 0);
+
+    if (paragraphs.length === 0) {
+        throw new Error("No extractable text found in PDF");
+    }
+
+    // Translate all paragraphs
+    const translations = await replicateTranslateBatch(paragraphs, targetLanguage);
+
+    // Build output PDF
+    const pdfDoc = await PDFDocument.create();
+    const font = await pdfDoc.embedFont(StandardFonts.Helvetica);
+    const boldFont = await pdfDoc.embedFont(StandardFonts.HelveticaBold);
+
+    const PAGE_WIDTH = 595;
+    const PAGE_HEIGHT = 842;
+    const MARGIN = 50;
+    const FONT_SIZE = 11;
+    const TITLE_SIZE = 13;
+    const LINE_HEIGHT = 16;
+    const MAX_LINE_CHARS = 80;
+
+    let page = pdfDoc.addPage([PAGE_WIDTH, PAGE_HEIGHT]);
+    let y = PAGE_HEIGHT - MARGIN;
+
+    function ensureSpace(needed: number) {
+        if (y - needed < MARGIN) {
+            page = pdfDoc.addPage([PAGE_WIDTH, PAGE_HEIGHT]);
+            y = PAGE_HEIGHT - MARGIN;
+        }
+    }
+
+    // Title
+    const title = `Translation to: ${targetLanguage}${sourceLanguage ? ` (from: ${sourceLanguage})` : ""}`;
+    ensureSpace(TITLE_SIZE + LINE_HEIGHT);
+    page.drawText(title, {
+        x: MARGIN,
+        y,
+        size: TITLE_SIZE,
+        font: boldFont,
+        color: rgb(0.2, 0.2, 0.7)
+    });
+    y -= TITLE_SIZE + LINE_HEIGHT;
+
+    // Draw translated paragraphs
+    for (const para of translations) {
+        const lines = wrapText(para, MAX_LINE_CHARS);
+        ensureSpace(lines.length * LINE_HEIGHT + LINE_HEIGHT);
+
+        for (const line of lines) {
+            ensureSpace(LINE_HEIGHT);
+            page.drawText(line, {
+                x: MARGIN,
+                y,
+                size: FONT_SIZE,
+                font,
+                color: rgb(0, 0, 0)
+            });
+            y -= LINE_HEIGHT;
+        }
+        y -= LINE_HEIGHT * 0.5; // paragraph gap
+    }
+
+    const pdfBytes = await pdfDoc.save();
+    return Buffer.from(pdfBytes);
+}