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.

package.json

@@ -127,6 +127,8 @@
     "agent-browser": "^0.6.0",
     "esbuild": "^0.27.3",
     "eslint": "^9.0.0",
+    "exceljs": "4.4.0",
+    "fflate": "0.8.2",
     "pixelmatch": "^6.0.0",
     "playwright": "^1.58.0",
     "pngjs": "^7.0.0",