feat(vue-docx): add showUpload and showDownload props

[Taozizz]

Summary

Adds showUpload and showDownload props to DocxViewer so read-only preview hosts can hide the upload and download entry points independently, instead of dropping the entire toolbar via showToolbar.

• Both props default to true, so existing behavior is unchanged.
• showUpload: false also hides the empty-state "Choose Word file" and error-state "Choose another Word file" buttons, since they open the same local file picker as the toolbar button.
• Naming follows the existing XlsxViewer showUpload convention.
• The demo page gains a props panel that exposes showToolbar, showUpload, showDownload, defaultThumbnailsOpen, isDark and defaultZoom as live toggles. defaultThumbnailsOpen is folded into the viewer key because it is only read on mount.

Commits

• feat(vue-docx): add showUpload and showDownload props — component, toolbar, docs, CHANGELOG, component tests
• feat(demo): add a props panel to the DocxViewer page — demo-only

Test plan

• Added component-test coverage in tests/component/vue-components.test.mjs: default visibility, both props hidden with the toolbar still rendering, and the error state with only showUpload disabled (download button retained).

Closes #2