[WIP] Fix all four Dependabot vulnerabilities (#12)

* Initial plan

* Initial analysis and plan for fixing 4 Dependabot security vulnerabilities

Co-authored-by: granatonatalia <[email protected]>

* Implement security fixes: CSP, security audit utility, xlsx→exceljs migration, axios/form-data overrides

Co-authored-by: granatonatalia <[email protected]>

* Fix TypeScript build errors by pinning compatible type definitions

Co-authored-by: granatonatalia <[email protected]>

* Address code review feedback: remove optional chaining and document CSP limitations

Co-authored-by: granatonatalia <[email protected]>

* Resolve merge conflicts with master branch

Co-authored-by: granatonatalia <[email protected]>

* Fix TypeScript error: add optional chaining to eachCell method

Co-authored-by: granatonatalia <[email protected]>

* Resolve merge conflicts with updated master branch (PRs #9-#13)

Co-authored-by: granatonatalia <[email protected]>

* Resolve merge conflicts (third iteration) - maintain type overrides and optional chaining

Co-authored-by: granatonatalia <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: granatonatalia <[email protected]>

Author: Copilot