Add complete playable chess game

[slugmandrew]

What changed

• replace the partial movement implementation with standards-compliant gameplay powered by chess.js
• add check, checkmate, draw detection, castling, en passant, and selectable promotion
• support both click-to-move and drag-and-drop through the same validated move path
• add a computer opponent that plays Black with easy, medium, and hard alpha-beta search depths
• add a visible thinking state, mode-aware input locking, and full-turn undo against the computer
• constrain the board to its layout column and enforce an 8×8 square grid so the outer rank and file remain visible
• add move history, captured pieces, undo, reset, legal-move highlights, and a responsive interface
• migrate from Create React App 4/Webpack 4 to Vite 4 and Vitest
• replace the generic README and add detailed architecture documentation
• remove unused prototype files and dependencies

Why

The existing UI rendered a chess board and supported some piece movement, but it did not enforce complete chess rules or reliably support playing a full game. It now supports both same-device play and games against a local computer opponent. The prior viewport-based board width could also overflow beside the information panel, cutting off the outer row and column; the board now sizes within its grid column.

Create React App 4's development server could not parse the public class fields shipped by chess.js 1.0. Vite pre-bundles the dependency with esbuild, resolving that parser failure while modernizing the development and test workflow.

Validation

• development server: pnpm start served the application and transformed chess.js successfully
• tests: 11 passing with pnpm test:run
• test coverage includes computer response flow, forced mate choice, full-turn undo, special chess moves, and click controls
• strict TypeScript and production build: pnpm build
• production bundle: 895 modules transformed by Vite 4.5.14
• diff whitespace validation: git diff --check