feat: Add book viewer at /book with shared design system

- Add html/index.html: book viewer with auto-discovering sidebar,
  prev/next navigation, keyboard shortcuts, and URL hash persistence
- Add html/book-page.css: shared stylesheet for all book pages derived
  from fabula-ultima-sheet.css (dark theme, CSS variables, Cinzel/
  Crimson Text fonts, common class styles)
- Add book.js entry point so webpack injects the shared CSS into the
  book viewer; update webpack.config.js for two entry points, split
  CSS chunk, CopyWebpackPlugin for book pages, and /book dev server
  rewrite rule
- Add scripts/strip_watermark.py: removes "Guest Customer (Order
  #52072168)" watermark artifacts from all 210 book pages
- Add scripts/restyle_book.py: strips per-page <style> blocks and
  injects <link rel="stylesheet" href="book-page.css"> into all pages
- Update Justfile deploy to scp -r dist/* for the new /book subtree

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

package.json

@@ -7,6 +7,7 @@
     "prettier": "^3.8.3"
   },
   "devDependencies": {
+    "copy-webpack-plugin": "^14.0.0",
     "css-loader": "^7.1.4",
     "css-minimizer-webpack-plugin": "^8.0.0",
     "html-webpack-plugin": "^5.6.7",