feat(preview): render XLSX spreadsheets in the file-preview overlay

xlsx files were download-only. Add a read-only, virtualized preview (sheet
tabs, number formats, merges, theme colours) parsed entirely in a browser
Web Worker with exceljs and fflate, loaded only when a spreadsheet is
opened. The workbook is checked against ZIP-bomb, entry and cell limits
before exceljs loads; cell text is written with textContent, formulas are
never evaluated and nothing referenced by the workbook is fetched. On the
server xlsx only joins the existing allowlist and classification, with a
10 MB cap on ?preview=true. xls and ods stay download-only.

src/web/public/index.html

@@ -3766,5 +3766,6 @@
   <script defer src="ultracode-windows.js"></script>
   <script defer src="session-lineage.js"></script>
   <script defer src="image-input.js"></script>
+  <script defer src="spreadsheet-preview.js"></script>
 </body>
 </html>