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>

package.json

@@ -7,6 +7,7 @@
     "@babel/core": "^7.29.7",
     "@babel/preset-react": "^7.29.7",
     "@babel/register": "^7.29.7",
+    "babel-loader": "^10.1.1",
     "copy-webpack-plugin": "^14.0.0",
     "css-loader": "^7.1.4",
     "css-minimizer-webpack-plugin": "^8.0.0",