Migrate Tailwind CSS v3 to v4 CSS-first architecture

Replace JS config with CSS-native @theme, @Utility, and @source directives.
Remove postcss/autoprefixer dependencies in favor of @tailwindcss/cli.
Convert cross-referenced base classes (focus-ring, btn, nav-item, toast) to
@Utility so @apply works correctly in v4. Remove 12 unused component classes.

Author: phmatray