refactor(client): Phase 6 decompose document components and enhance FileThumbnail

- Extract useDocumentUpload hook (96L) for file upload mutation + validation
- Extract useDocumentDelete hook (42L) for delete mutation (deduplicated)
- Extract useDocumentPreview hook (38L) for lightbox state management
- Create DocumentPreviewTrigger shared accessible click wrapper (38L)
- Refactor document-file-item (227→165L, -27%), document-viewer (598→562L),
  questionnaire-document-preview (261→220L) to use new hooks
- Enhance FileThumbnail with onPreview (eye icon overlay), variant prop
  (thumbnail/icon/detailed), and skeleton loader with spinner
- FileThumbnail now accepts optional file prop for null-safe rendering
- Add lightbox preview to employee profile personnel photo
- Update barrel exports in documents/index.ts