refactor: replace exceljs with write-excel-file for analytics export

exceljs (latest 4.4.0) ships years-old transitive dependencies that
npm flags as deprecated on every install (rimraf 2, glob 7, inflight,
fstream, lodash.isequal) and needed a uuid override for a security
advisory. It was only used for the single-sheet analytics XLSX export,
which write-excel-file covers with the same output (column widths,
bold/grey header, formula-injection escaping).

Removes the now-unneeded exceljs uuid override.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/routes/analytics.js

@@ -1,6 +1,6 @@
 import { Router, json } from 'express';
 import crypto from 'crypto';
-import ExcelJS from 'exceljs';
+import writeXlsxFile from 'write-excel-file/node';
 import PDFDocument from 'pdfkit';
 import { getDb } from '../config/database.js';
 import { authenticateToken } from '../middleware/auth.js';
@@ -246,26 +246,24 @@ router.get('/:id/export/:format', authenticateToken, async (req, res) => {
 
     if (format === 'xlsx') {
       // Prefix-escape strings that would otherwise be evaluated as formulas.
-      const sanitizeXlsx = (r) => {
-        const out = {};
-        for (const k of Object.keys(r)) {
-          const v = r[k];
-          out[k] = (typeof v === 'string' && /^[=+\-@\t\r]/.test(v)) ? "'" + v : v;
-        }
-        return out;
-      };
-      const workbook = new ExcelJS.Workbook();
-      const sheet = workbook.addWorksheet('Analytics');
-      sheet.columns = COLUMNS;
-      rows.forEach(r => sheet.addRow(sanitizeXlsx(r)));
-      // Style header row
-      sheet.getRow(1).font = { bold: true };
-      sheet.getRow(1).fill = { type: 'pattern', pattern: 'solid', fgColor: { argb: 'FFE0E0E0' } };
+      const sanitizeXlsx = (v) =>
+        (typeof v === 'string' && /^[=+\-@\t\r]/.test(v)) ? "'" + v : v;
+
+      const headerRow = COLUMNS.map(c => ({
+        value: c.header,
+        fontWeight: 'bold',
+        backgroundColor: '#e0e0e0',
+      }));
+      const dataRows = rows.map(r => COLUMNS.map(c => ({ value: sanitizeXlsx(r[c.key]) })));
+
+      const buffer = await writeXlsxFile([headerRow, ...dataRows], {
+        columns: COLUMNS.map(c => ({ width: c.width })),
+        sheet: 'Analytics',
+      }).toBuffer();
 
       res.setHeader('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
       res.setHeader('Content-Disposition', `attachment; filename="${safeName}.xlsx"`);
-      await workbook.xlsx.write(res);
-      return res.end();
+      return res.send(buffer);
     }
 
     if (format === 'pdf') {