refactor: Convert book viewer to idiomatic React

Replace the SSR + injected vanilla-JS navigation script with a proper
client-side React app. BookIndex now uses useState/useEffect/useCallback
for all navigation state, scroll tracking, keyboard shortcuts, and history
management. webpack.config.js switches from renderToStaticMarkup to
embedding page data as window.__BOOK_DATA__ JSON; book.js becomes a
createRoot entry point. Also adds babel-loader for JSX bundling, fixes
#root display:contents so the flex height chain is preserved, and
restores missing CSS for header, .logo, .toolbar, and .tab buttons.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

book.js

@@ -1 +1,8 @@
-import './fabula-ultima-sheet.css';
+import React from 'react';
+import { createRoot } from 'react-dom/client';
+import BookIndex from './src/BookIndex.jsx';
+
+const { title, logoText, pages } = window.__BOOK_DATA__;
+createRoot(document.getElementById('root')).render(
+  <BookIndex title={title} logoText={logoText} pages={pages} />
+);