fix(central): no-issue: fix ESM build reference data export regression

[tcaiger]

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After the build-less switch (#10097), the central server runs xlsx's ESM build directly. xlsx's ESM build does not bind Node's fs automatically, so readFile/writeFile throw cannot save file — breaking reference-data export (and any disk-backed import).

Fix: call XLSX.set_fs(fs) once at process start (app/index.js, the universal entrypoint for the server and all subcommands), plus in the export writer util (excelUtils.js) as a safety net for direct/test use. No behaviour change beyond enabling file IO.

Reproduced via GET /api/admin/export/referenceData returning a 500 (cannot save file ./export-*.xlsx); fixed after restart.

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Fix these issues identified on the pull request. One commit per issue fixed.

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packages/central-server/app/utils/excelUtils.js:5: XLSX.set_fs(fs) is called here at module initialisation AND again in index.js. Due to ESM semantics, module bodies of dependencies execute before the importing module's body, so this call in excelUtils.js will always run before the one in index.js. The comment in index.js ('bind it once at process start') is therefore misleading — it actually runs last. More importantly, any module that uses xlsx directly without importing excelUtils.js won't have the binding applied until index.js's body runs. Pick one canonical place: if the intent is a guaranteed early binding, keep only the index.js call (it will run before any user-initiated xlsx operations, just not before dependency module bodies). If the intent is defensive per-module binding, remove the index.js call and add the same pattern to every file that uses xlsx. Having both creates maintenance confusion about which is the real fix.

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