Update multiple UI panels to use skeleton components instead of loading text or spinners.
Standardize loading states in AudiobookExportModal, AdminFeaturesPanel, AdminProvidersPanel,
DocumentSelectionModal, and SegmentsSidebar for improved visual consistency and user feedback.
Move shared button, input, and listbox styles to a new formPrimitives module
used across admin panels, settings, and document settings. Update admin and
settings components to import these styles from the new location. Refactor
DocumentSelectionModal to use a direct files prop with loading and error
states, removing fetcher logic and internal fetch state. Add a custom hook
for library document queries. Clean up legacy admin/ui primitives and
standardize segmented controls and section layouts.
