Fix CI build failure and add missing LICENSE files

Add "require" export condition to tailwind-preset for Node 18 CJS
compatibility (jiti resolution). Add LICENSE files to react,
tailwind-preset, and hex-background packages. Add license field to
showcase packages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/tailwind-preset/package.json

@@ -7,7 +7,8 @@
   "exports": {
     ".": {
       "types": "./dist/index.d.ts",
-      "import": "./dist/index.js"
+      "import": "./dist/index.js",
+      "require": "./dist/index.js"
     }
   },
   "main": "dist/index.js",