feat(reporting): add CSV and XLSX export to findings table

Adds an Export dropdown button to the Reporting page action bar.
Exports respect current filters, sort order, and column visibility.
CSV uses pure JS (UTF-8 BOM for Excel compatibility); XLSX uses SheetJS
with auto-fitted column widths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -12,7 +12,8 @@
     "react-dom": "^19.2.4",
     "react-markdown": "^10.1.0",
     "react-scripts": "5.0.1",
-    "web-vitals": "^2.1.4"
+    "web-vitals": "^2.1.4",
+    "xlsx": "^0.18.5"
   },
   "scripts": {
     "start": "react-scripts start",