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>

.githooks/pre-commit

@@ -0,0 +1,6 @@
+#!/bin/sh
+# Run TypeScript type-checking before each commit.
+# Babel strips types without checking them, so this is what actually
+# enforces type correctness. Bypass with `git commit --no-verify`.
+
+npm run typecheck