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.
