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

src/components/Dock.tsx

@@ -25,6 +25,7 @@ function PdfDock() {
   const timer = useRef<ReturnType<typeof setTimeout>>(undefined);
 
   useEffect(() => {
+    if (!book) return;
     clearTimeout(timer.current);
     setBusy(true);
     timer.current = setTimeout(() => {
@@ -44,7 +45,7 @@ function PdfDock() {
     <section className="dock">
       <div className="dock-head">
         <span className="label">Preview</span>
-        <span className="meta">{busy ? "updating…" : TRIM_LABEL[book.settings.trim]}</span>
+        <span className="meta">{busy ? "updating…" : TRIM_LABEL[book?.settings.trim ?? "6x9"]}</span>
       </div>
       {error ? (
         <pre className="dock-error">{error}</pre>
@@ -58,19 +59,21 @@ function PdfDock() {
 }
 
 function HtmlDock() {
-  const chapters = useBook((s) => s.book.chapters);
-  const trim = useBook((s) => s.book.settings.trim);
+  const book = useBook((s) => s.book);
   const activeChapterId = useBook((s) => s.activeChapterId);
+  const chapters = book?.chapters ?? [];
   const idx = chapters.findIndex((c) => c.id === activeChapterId);
-  const chapter = chapters[idx];
-  const html = useMemo(() => generateHTML(chapter.content, editorExtensions), [chapter.content]);
+  const chapter = chapters[idx] ?? chapters[0];
+  const html = useMemo(() => (chapter ? generateHTML(chapter.content, editorExtensions) : ""), [chapter]);
+
+  if (!book || !chapter) return null;
 
   return (
     <section className="dock">
       <div className="dock-head">
         <span className="label">Preview</span>
         <span className="meta">
-          {TRIM_LABEL[trim]} · {idx + 1} of {chapters.length}
+          {TRIM_LABEL[book.settings.trim]} · {idx + 1} of {chapters.length}
         </span>
       </div>
       <div className="page">

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>
   );

src/components/Sidebar.tsx

@@ -2,17 +2,18 @@ import { useBook } from "../store/useBook";
 import { Icon } from "./Icon";
 
 export function Sidebar() {
-  const chapters = useBook((s) => s.book.chapters);
+  const chapters = useBook((s) => s.book?.chapters ?? []);
   const activeChapterId = useBook((s) => s.activeChapterId);
   const setActiveChapter = useBook((s) => s.setActiveChapter);
   const addChapter = useBook((s) => s.addChapter);
+  const closeBook = useBook((s) => s.closeBook);
 
   return (
     <aside className="sidebar">
-      <div className="brand">
+      <button className="brand" onClick={closeBook} title="Back to library">
+        <Icon d="M14 7l-5 5 5 5" size={15} />
         <span className="mark">margin</span>
-        <span className="dot" />
-      </div>
+      </button>
       <div className="nav-label">Chapters</div>
       <ul className="chapters">
         {chapters.map((chapter, i) => (