chore(all): Replace exceljs with write-excel-file

[tombrunet]

Summary

Replace the peer dependency (unmaintained) with across all packages. The XLSX report output is functionally equivalent — same five sheets, same column widths, same colour coding and cell styling.

Motivation

exceljs has not received meaningful maintenance. Switching to write-excel-file removes the dependency on an unmaintained library and eliminates the 'use strict' patching workaround that was required in the browser extension's preprocess.js.

Changes

Core logic — common/module/src/report/ACReporterXLSX.ts

• Full rewrite from ExcelJS's imperative workbook/worksheet/cell API to write-excel-file's data-array model.
• Each sheet builder now returns a { data, sheet, columns } object; all sheets are passed to writeExcelFile(sheets).toFile(filename) in one call.
• Cell colours converted from ExcelJS ARGB format ("FF403151") to standard 6-digit hex ("#403151").
• Lazy-load guard updated: dynamic import now targets write-excel-file/node; warning message updated accordingly.
• worksheet.commit() / workbook.commit() calls removed (not needed by the new library).

Extension — accessibility-checker-extension/

• package.json: replaced exceljs direct dependency with write-excel-file.
• preprocess.js: removed the sed commands that stripped 'use strict' from the exceljs bundle (no longer needed).
• jest.config.js: removed stale comment referencing exceljs.

E2E test helper — accessibility-checker-extension/test/

• support/steps/steps_excel.ts: replaced exceljs Workbook reader with read-excel-file/node; updated cell-address parsing to use the library's row/column array model.
• package.json: replaced exceljs with read-excel-file.

package.json peer / direct dependency updates

All packages that declared exceljs as a peer or direct dependency have been updated to declare write-excel-file@^4.1.1 instead:

• accessibility-checker/package.json
• accessibility-checker/src/package.json
• cypress-accessibility-checker/package.json + package/package.json
• vitest-accessibility-checker/package.json + src/package.json + package/package.json
• karma-accessibility-checker/package.json
• common/module/package.json

Pre-built JS artefacts regenerated

All committed copies of ACReporterXLSX.js (CJS + MJS outputs across accessibility-checker, cypress-accessibility-checker, vitest-accessibility-checker, karma-accessibility-checker) were rebuilt from the new TypeScript source and committed.

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Testing checklist

Confirm these scenarios before merging.

XLSX report output (accessibility-checker Node.js)

• Run accessibility-checker against a page with violations and confirm a .xlsx file is written to disk.
• Open the generated file — verify 5 sheets are present: Overview, Scan summary, Issue summary, Issues, Definition of fields.
• Overview sheet: confirm title row background is dark purple (#403151), summary counts (Violations / Needs review / Recommendations / Archived) are correct, and column widths are reasonable.
• Scan summary sheet: confirm one data row per scan, all 9 columns populated, numeric columns (Violations, Needs review, …) are right-aligned with borders.
• Issue summary sheet: confirm Level 1–4 sections are present with correct counts; rows within each level are sorted descending by count.
• Issues sheet: confirm one row per issue, all 14 columns populated, header row has dark purple background.
• Definition of fields sheet: confirm both the Scan summary and Issues definition tables are present.
• Run a scan with outputFilenameTimestamp: false and confirm the output file is named results.xlsx (not timestamped).
• Confirm that if write-excel-file is not installed the warning "Warning: write-excel-file is not installed…" is printed and the process does not crash.

Browser extension (accessibility-checker-extension)

• Build the extension (npm run build:dev) — confirm it compiles without errors (no exceljs webpack error or 'use strict' conflict).
• Load the extension in Chrome/Firefox and generate an XLSX report from the UI — confirm the file downloads and opens correctly in Excel / LibreOffice.
• Confirm preprocess.js no longer modifies any node_modules files on build.

E2E tests (accessibility-checker-extension/test)

• Run npm test in accessibility-checker-extension/test — the Cucumber step Excel Sheet "…" Cell "…" is "…" should pass using read-excel-file.

Downstream packages

• cypress-accessibility-checker: run npm run test and confirm XLSX output is produced correctly.
• vitest-accessibility-checker: run npm run test and confirm XLSX output is produced correctly.
• karma-accessibility-checker: run npm run build with no errors.