Add hex background to showcase with interactive controls

- Render HexGridBackground globally on every page
- Add dedicated Hex Background page with sliders for all 8 props
- Lift hex props state to App for real-time global updates
- Add three, @react-three/fiber, hex-background deps

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

packages/showcase/desktop/package.json

@@ -25,12 +25,15 @@
     "clean": "rm -rf dist release"
   },
   "dependencies": {
+    "@dangerousthings/hex-background": "^0.2.0",
     "@dangerousthings/react": "*",
     "@dangerousthings/tokens": "*",
     "@dangerousthings/web": "*",
+    "@react-three/fiber": "^9.5.0",
     "react": "^18.0.0",
     "react-dom": "^18.0.0",
-    "react-icons": "^5.6.0"
+    "react-icons": "^5.6.0",
+    "three": "^0.183.2"
   },
   "devDependencies": {
     "@dangerousthings/tailwind-preset": "*",