refactor: Move source files from root into src/

Relocates book.js, fabula-ultima-sheet.css, and fabula-ultima-sheet.html
into src/ to consolidate all source files under one directory. Updates
webpack entry points, HTML template path, and CSS import accordingly.

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

webpack.config.js

@@ -45,7 +45,7 @@ module.exports = (env, argv) => {
   return {
     entry: {
       sheet: "./src/sheet-main.jsx",
-      book: "./book.js",
+      book: "./src/book.js",
     },
     output: {
       filename: isProd ? "[name].[contenthash].js" : "[name].js",
@@ -80,7 +80,7 @@ module.exports = (env, argv) => {
         ? [new MiniCssExtractPlugin({ filename: "[name].[contenthash].css" })]
         : []),
       new HtmlWebpackPlugin({
-        template: "./fabula-ultima-sheet.html",
+        template: "./src/fabula-ultima-sheet.html",
         filename: "index.html",
         chunks: ["sheet"],
         scriptLoading: "blocking",