feat(xlsx-viewer): expose getCellStyle for custom per-cell styling

[tothienbao6a0]

Summary

Forwards the new getCellStyle prop from @extend-ai/react-xlsx through the @extend/xlsx-viewer registry component, so consumers can apply custom per-cell styling (highlights, outlines, status tints) on top of the workbook's own Excel formatting — without forking the component.

• Threads getCellStyle through XlsxViewerPreview → XlsxViewerContent → XlsxWorkbookLoadedViewer → XlsxWorkbookSurface → XlsxViewer, typed via XlsxViewerProps["getCellStyle"] so the signature stays in sync with the underlying package.
• Bumps the @extend-ai/react-xlsx dependency (package.json + registry.json) to the release that adds the prop.
• Documents getCellStyle in the component's API reference.

Dependency / status

Draft — blocked on getCellStyle shipping in a published @extend-ai/react-xlsx release (upstream: extend-hq/react-xlsx#8). The version pin here is provisional and should be matched to whatever release includes that change. Ready to mark for review once the package is published.

Test plan

• pnpm typecheck (verified locally against a getCellStyle-enabled build of the package)
• pnpm registry:validate
• prettier --check on changed files
• pnpm build once the dependency release is available

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