fix(persist): schema-version guard and editor content check

A book file's declared schema was never read, so a file from a newer
app version would be opened, normalized and autosaved over in the old
format. Refuse to open books whose schema is newer than supported.
Enable the editor content check so chapters with unreadable content
warn the user instead of silently loading stripped content that the
next edit would persist.

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

src/editor/Editor.tsx

@@ -10,11 +10,14 @@ interface EditorProps {
   content: JSONContent;
   onChange: (content: JSONContent) => void;
   onReady: (editor: TiptapEditor | null) => void;
+  onContentError: (error: Error) => void;
 }
 
-export function Editor({ bookId, chapterId, content, onChange, onReady }: EditorProps) {
+export function Editor({ bookId, chapterId, content, onChange, onReady, onContentError }: EditorProps) {
   const onChangeRef = useRef(onChange);
   onChangeRef.current = onChange;
+  const onContentErrorRef = useRef(onContentError);
+  onContentErrorRef.current = onContentError;
 
   const latest = useRef<ChapterPosition | null>(null);
 
@@ -22,7 +25,9 @@ export function Editor({ bookId, chapterId, content, onChange, onReady }: Editor
     extensions: editorExtensions,
     content,
     immediatelyRender: false,
+    enableContentCheck: true,
     editorProps: { attributes: { class: "prose" } },
+    onContentError: ({ error }) => onContentErrorRef.current(error),
     onUpdate: ({ editor }) => onChangeRef.current(editor.getJSON()),
   });