refactor(registry): split RegistryPkgShow.vue into modular pkgshow components

- Extract header into PkgShowHeader.vue
- Extract overview tab into PkgOverviewTab.vue
- Extract docs tab into PkgDocsTab.vue
- Extract files tab into PkgFilesTab.vue
- Extract preview modal into PkgPreviewModal.vue
- Extract versions tab into PkgVersionsTab.vue
- Keep all business logic in parent component
- No behavior change, purely structural refactor

Author: GaspardKirira