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-lock.json

@@ -8,6 +8,7 @@
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         "@babel/register": "^7.29.7",
+        "babel-loader": "^10.1.1",
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         "css-loader": "^7.1.4",
         "css-minimizer-webpack-plugin": "^8.0.0",
@@ -1850,6 +1851,97 @@
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+        "node": ">=10"
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+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
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+    "node_modules/babel-loader/node_modules/p-locate": {
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     "node_modules/baseline-browser-mapping": {
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       "dev": true,
       "license": "ISC"
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+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=10"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
     }
   }
 }