📎 fix: Keep SharePoint Picker Selections While Navigating (#15389)

The picker never set `selection.enablePersistence`, so it dropped the current
selection every time the user opened another folder or switched pivot. A selection
could only ever be assembled from a single location — pick three files, open another
folder, and the three are gone.

One option on the picker configuration; no change to what LibreChat does with the
result.

Fixes the selection half of #15237. Folder selection — the other half — is a new
capability rather than a fix, and is parked on `feat/sharepoint-folder-selection`.

client/src/hooks/Files/useSharePointPicker.ts

@@ -285,6 +285,10 @@ export default function useSharePointPicker({
         },
         selection: {
           mode: 'multiple',
+          /** Without this the picker drops the current selection every time the user
+           * opens another folder or switches pivot, so a selection can only ever be
+           * assembled from one location. */
+          enablePersistence: true,
           maximumCount: maxSelectionCount,
         },
         title: localize('com_files_sharepoint_picker_title'),