2.0: Upgrade CMS to GP247 core v2 (Laravel 13, Tailwind 4)

Move to gp247/core:^2.0 + gp247/front:^2.0 on Laravel 13 / PHP 8.3,
mirroring the s-cart root repo (CMS = core + front, no shop).

- composer.json: bump php/framework/tinker, sync require-dev, add
  laravel/pao, drop laravel/sail, add setup/test scripts + authors
- bootstrap/app.php: Application::configure() style, gp247 exception report
- Tailwind v3 -> v4: @tailwindcss/vite, @import in app.css, drop
  tailwind.config.js / postcss.config.js / resources/js/bootstrap.js
- Remove committed v1 app/GP247 admin controllers (live in vendor in v2)
- Add config/trustedproxy.php; rename cms:info / cms:update signatures
- phpunit.xml, .gitignore, .env.example refreshed to L13 baseline
- README (en/vi): v2 tags, controller-override mechanism, src/routes.php,
  ASCII logo, EN/VI language switcher, drop screenshot image

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