refactor: Migrate the app to TypeScript

Convert the React app from JS/JSX to TS/TSX and add type-checking:
- Rename sheet-main, book, BookIndex, CharacterSheet to .ts(x) and
  add types (Fields, Bond, ClassEntry, Spell, CheckMap; loose
  SavedData for abbreviated save/share payloads)
- Add globals.d.ts for CSS imports and the __BOOK_DATA__ global
- tsconfig.json (strict, noEmit) and a 'typecheck' npm script
- webpack: handle ts/tsx via @babel/preset-typescript
- Enforce types with a tracked pre-commit hook (core.hooksPath),
  wired up automatically via the 'prepare' script
- Update stale Justfile format target for the src/ layout

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

webpack.config.js

@@ -44,8 +44,8 @@ module.exports = (env, argv) => {
 
   return {
     entry: {
-      sheet: "./src/sheet-main.jsx",
-      book: "./src/book.js",
+      sheet: "./src/sheet-main.tsx",
+      book: "./src/book.tsx",
     },
     output: {
       filename: isProd ? "[name].[contenthash].js" : "[name].js",
@@ -56,11 +56,13 @@ module.exports = (env, argv) => {
     module: {
       rules: [
         {
-          test: /\.(js|jsx)$/,
+          test: /\.(js|jsx|ts|tsx)$/,
           exclude: /node_modules/,
           use: {
             loader: "babel-loader",
-            options: { presets: ["@babel/preset-react"] },
+            options: {
+              presets: ["@babel/preset-react", "@babel/preset-typescript"],
+            },
           },
         },
         {
@@ -73,7 +75,7 @@ module.exports = (env, argv) => {
       ],
     },
     resolve: {
-      extensions: [".js", ".jsx"],
+      extensions: [".ts", ".tsx", ".js", ".jsx"],
     },
     plugins: [
       ...(isProd