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>

package.json

@@ -19,7 +19,6 @@
     "concurrently": "^9.0.0",
     "cors": "^2.8.5",
     "dotenv": "^17.3.1",
-    "exceljs": "^4.4.0",
     "express": "^5.2.1",
     "express-rate-limit": "^8.5.2",
     "flatpickr": "^4.6.13",
@@ -38,6 +37,7 @@
     "react-hot-toast": "^2.4.0",
     "react-router-dom": "^7.15.1",
     "uuid": "^14.0.0",
+    "write-excel-file": "^4.1.1",
     "xml2js": "^0.6.0"
   },
   "devDependencies": {
@@ -49,11 +49,6 @@
     "tailwindcss": "^4.3.0",
     "vite": "^8.0.0"
   },
-  "overrides": {
-    "exceljs": {
-      "uuid": "^11.1.1"
-    }
-  },
   "allowScripts": {
     "better-sqlite3@12.11.1": true
   }