fix: accept border style="none" in loadWorkbook (#99)

[baseballyama]

Summary

loadWorkbook rejected any workbook whose border sides use style="none", throwing expected one of [thin, medium, ...]; got "none". none is the first value of ECMA-376 §18.18.3 ST_BorderStyle (no stroke), so it is valid input and is now accepted.

Motivation

OnlyOffice — and other producers — write the no-border value explicitly as <left style="none"/>. Excel omits the side element in that case, which is why this went unnoticed until a file from OnlyOffice hit it. The value was simply missing from the SideStyle enumeration.

Closes #99

Changes

• @office-kit/xlsx/styles: SideStyle (and SIDE_STYLES) now include 'none'. Sides with style="none" parse, round-trip faithfully, and draw no stroke in HTML/SVG preview.

Testing

• Added tests/styles/issue-99.test.ts — a failing-first test that parses <side style="none"/> and exercises makeSide({ style: 'none' }).
• Verified end-to-end against the reporter's attached lookup_values_lut.xlsx (5 sides with style="none"): it threw before this change and loads cleanly after, with none preserved on round-trip.
• pnpm typecheck && pnpm lint && pnpm test green.

Reproduce:

pnpm vitest run tests/styles/issue-99.test.ts

Breaking changes

None. This widens accepted input; existing values are unaffected.

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a breaking change, I have added a changeset / CHANGELOG entry and flagged it above.
• I have re-read my own diff and removed dead code, debug prints, and stale comments.
• If I used an LLM to draft this PR, I have verified each change myself, the PR represents real work that warrants a maintainer's review, and I am willing to defend each line in review.

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