feat(ui): back-to-library, export-only download button, save in book setup

src/components/Settings.tsx

@@ -28,12 +28,14 @@ function Field({ label, children }: { label: string; children: ReactNode }) {
   );
 }
 
-export function Settings({ onClose }: { onClose: () => void }) {
-  const metadata = useBook((s) => s.book.metadata);
-  const settings = useBook((s) => s.book.settings);
+export function Settings({ onClose, onSave }: { onClose: () => void; onSave: () => void }) {
+  const book = useBook((s) => s.book);
   const setMetadata = useBook((s) => s.setMetadata);
   const setSettings = useBook((s) => s.setSettings);
 
+  if (!book) return null;
+  const { metadata, settings } = book;
+
   return (
     <div className="overlay" onClick={onClose}>
       <div className="panel" onClick={(e) => e.stopPropagation()}>
@@ -81,6 +83,17 @@ export function Settings({ onClose }: { onClose: () => void }) {
             Add bleed for full-page images (print)
           </label>
         </div>
+        <div className="panel-foot">
+          <button
+            className="btn-primary"
+            onClick={() => {
+              onSave();
+              onClose();
+            }}
+          >
+            Save
+          </button>
+        </div>
       </div>
     </div>
   );