feat: migrate from @iconify/tailwind to @iconify/tailwind4 (#8724)

## Summary

Migrate from `@iconify/tailwind` (Tailwind 3 JS plugin) to
`@iconify/tailwind4` (native Tailwind 4 CSS plugin), moving all config
into CSS directives.

## Changes

- **What**: Replace `addDynamicIconSelectors()` JS plugin with `@plugin
"@iconify/tailwind4"` CSS directive. Move `boxShadow` theme extension
into `@theme` block. Delete both `tailwind.config.ts` files and the
runtime `iconCollection.ts` module.
- **Dependencies**: `@iconify/tailwind` removed, `@iconify/tailwind4`
added

## Review Focus

- `from-folder` path resolution in monorepo context (paths relative to
project root)
- SVG auto-cleanup behavior of `from-folder` vs the previous manual
`iconCollection.ts` loader
- Removal of `@config` directive and both tailwind config files — all
config now in CSS

## Files

| File | Change |
|------|--------|
| `pnpm-workspace.yaml` | Swap catalog entry |
| `packages/design-system/package.json` | Swap dep, remove
`tailwind-config` export |
| `packages/design-system/src/css/style.css` | Add `@plugin`,
`--shadow-interface` theme token, remove `@config` |
| `packages/design-system/tailwind.config.ts` | Deleted |
| `packages/design-system/src/iconCollection.ts` | Deleted |
| `tailwind.config.ts` | Deleted |
| `tsconfig.json`, `components.json` | Remove stale references |
| `knip.config.ts` | Ignore `@iconify-json/lucide` (CSS-consumed, not
JS-imported) |
| Docs | Updated `CONTRIBUTING.md` and `icons/README.md` |

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8724-feat-migrate-from-iconify-tailwind-to-iconify-tailwind4-3006d73d36508144a9b3e7ae73448f98)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: GitHub Action <action@github.com>

packages/design-system/src/css/style.css

@@ -7,11 +7,16 @@
 
 @plugin 'tailwindcss-primeui';
 
-@config '../../tailwind.config.ts';
+@plugin "@iconify/tailwind4" {
+  scale: 1.2;
+  icon-sets: from-folder(comfy, './packages/design-system/src/icons');
+}
 
 @custom-variant touch (@media (hover: none));
 
 @theme {
+  --shadow-interface: var(--interface-panel-box-shadow);
+
   --text-xxs: 0.625rem;
   --text-xxs--line-height: calc(1 / 0.625);