Display AI recommendation reason in modal

Passes aiReason from the BookDate page into AudiobookDetailsModal and updates the modal to accept an optional aiReason prop (string | null). When provided, the modal renders a titled section "Why This Was Recommended" with styled content above the details grid. This includes prop/interface changes and a default value to preserve existing behavior when no reason is available.

src/app/bookdate/page.tsx

@@ -415,6 +415,7 @@ export default function BookDatePage() {
             isAvailable={currentRec.isAvailable}
             requestedByUsername={currentRec.requestedByUsername}
             hideRequestActions
+            aiReason={currentRec.aiReason}
           />
         ) : null;
       })()}