feat: restore static CSS extraction for production builds

[CharlesCoeder]

Summary

• The withTamagui webpack plugin was removed from apps/next/next.config.js in 063c8b1 when the repo switched to Turbopack
• This restores static CSS extraction using the Tamagui CLI (tamagui build), which works with Turbopack
• Aligns with the recommended v2 approach per the Next.js guide — using the CLI for build-time optimization instead of the webpack plugin
• Cleans up the now-unused DISABLE_EXTRACTION env var references from turbo.json and web:extract script

What changed

• apps/next/tamagui.build.ts — new build config for the Tamagui CLI
• apps/next/package.json — build script wraps next build with tamagui build --target web
• apps/next/app/layout.tsx — imports the generated CSS
• apps/next/public/tamagui.generated.css — generated static CSS (558 lines)
• package.json — updated web:extract script to use the new approach
• turbo.json — removed DISABLE_EXTRACTION from env lists

Results

• 14/18 components optimized, 12 flattened to static CSS
• Dev mode unchanged (Turbopack runtime, no extraction)
• Production builds get static CSS extraction via the CLI pre-build step