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>

package.json

@@ -1,12 +1,17 @@
 {
   "scripts": {
     "build": "webpack --mode=production",
-    "dev": "webpack serve --mode=development"
+    "dev": "webpack serve --mode=development",
+    "typecheck": "tsc --noEmit",
+    "prepare": "git config core.hooksPath .githooks || true"
   },
   "devDependencies": {
     "@babel/core": "^7.29.7",
     "@babel/preset-react": "^7.29.7",
+    "@babel/preset-typescript": "^7.29.7",
     "@babel/register": "^7.29.7",
+    "@types/react": "^19.2.17",
+    "@types/react-dom": "^19.2.3",
     "babel-loader": "^10.1.1",
     "copy-webpack-plugin": "^14.0.0",
     "css-loader": "^7.1.4",
@@ -17,6 +22,7 @@
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
     "style-loader": "^4.0.0",
+    "typescript": "^6.0.3",
     "webpack": "^5.107.2",
     "webpack-cli": "^7.0.3",
     "webpack-dev-server": "^5.2.4"