Migrate to Lightning CSS

Replace the current PostCSS pipeline with Lightning CSS. Lightning CSS is a fast CSS parser, transformer, and minifier written in Rust, with built-in support for nesting, custom media queries, and modern CSS features without needing separate plugins.

Motivation

• Significantly faster CSS processing (Rust-based)
• Native support for CSS nesting, no plugin needed
• Built-in vendor prefixing and browser target support
• Vite has built-in Lightning CSS integration
• Reduce dependency count by removing PostCSS plugins

Tasks

• Enable css.lightningcss in vite.config.js
• Replace postcss-nested-props usage with standard CSS nesting
• Replace postcss-simple-vars with CSS custom properties
• Replace postcss-easy-import with native CSS imports
• Remove postcss-preset-env (Lightning CSS handles transforms)
• Remove postcss.config.cjs
• Update svelte-preprocess config if needed
• Test all component styles for regressions

Risks

• postcss-nested-props shorthand has no direct equivalent in Lightning CSS — need to expand manually
• postcss-simple-vars ($var syntax) needs migrating to var(--var)
• Some edge cases in nesting behavior may differ between PostCSS and Lightning CSS