fix(library): surface unreadable books

list_books silently skipped any .margin file it could not read, parse,
or that lacked an id, so a truncated manuscript just vanished. Emit a
corrupt placeholder entry instead and show it in the library as a
non-openable card with a recovery hint pointing at the .bak backup.

Claude-Session: https://claude.ai/code/session_01RzTWrLiy4zGuw2hZSQGamk

src/components/Library.tsx

@@ -78,17 +78,25 @@ export function Library({ onOpen }: { onOpen: (book: Book) => void }) {
             <span className="card-badge">Example</span>
           </button>
         )}
-        {books.map((b) => (
-          <div
-            key={b.id}
-            className="card card-book"
-            onClick={() => loadBook(b.id).then(onOpen).catch((e) => setNotice(`Could not open book: ${e}`))}
-          >
-            <span className="card-title">{b.title || "Untitled"}</span>
-            {b.author && <span className="card-author">{b.author}</span>}
-            <RowMenu label="Book options" className="card-menu" onDelete={() => setPendingDelete(b)} />
-          </div>
-        ))}
+        {books.map((b) =>
+          b.corrupt ? (
+            <div key={b.id} className="card card-book card-corrupt">
+              <span className="card-title">{b.title}</span>
+              <span className="card-author">Couldn't be read; a .bak backup may sit beside it.</span>
+              <RowMenu label="Book options" className="card-menu" onDelete={() => setPendingDelete(b)} />
+            </div>
+          ) : (
+            <div
+              key={b.id}
+              className="card card-book"
+              onClick={() => loadBook(b.id).then(onOpen).catch((e) => setNotice(`Could not open book: ${e}`))}
+            >
+              <span className="card-title">{b.title || "Untitled"}</span>
+              {b.author && <span className="card-author">{b.author}</span>}
+              <RowMenu label="Book options" className="card-menu" onDelete={() => setPendingDelete(b)} />
+            </div>
+          ),
+        )}
       </div>
       {notice && (
         <div className="toast" onClick={() => setNotice(null)}>

src/library.ts

@@ -6,6 +6,7 @@ export interface BookSummary {
   id: string;
   title: string;
   author: string;
+  corrupt?: boolean;
 }
 
 export async function listBooks(): Promise<BookSummary[]> {

src/styles/app.css

@@ -1209,6 +1209,16 @@ body.resizing {
   box-shadow: 0 4px 10px rgba(35, 32, 27, 0.1), 0 20px 38px rgba(35, 32, 27, 0.13);
 }
 
+.card-corrupt {
+  cursor: default;
+  opacity: 0.65;
+  border-style: dashed;
+}
+
+.card-corrupt:hover {
+  transform: none;
+}
+
 .card-title {
   font-family: var(--font-book);
   font-size: 20px;