fix(editor): isolate undo history per chapter

Key the editor by chapter id so each chapter gets a fresh TipTap
instance with its own history. Previously a single instance was reused
across chapters, so the content swap entered the undo stack and Ctrl+Z
after switching restored the prior chapter's doc into the current one,
which autosave then persisted.

With remount, save the cursor/scroll position on unmount (from a ref,
not the editor, which is destroyed by then) and load it on mount,
replacing the in-place chapter-switch bookkeeping.

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

src/components/EditorView.tsx

@@ -263,6 +263,7 @@ export function EditorView() {
                   )}
                 </header>
                 <Editor
+                  key={chapter.id}
                   bookId={book.id}
                   chapterId={chapter.id}
                   content={chapter.content}