style: replace emoji heart with primary Heart icon and polish Valknar link

- Swap 💜 for a filled Lucide Heart icon in text-primary in both Footer and AppSidebar
- Style Valknar link with animated underline (decoration-primary on hover)
- Add sidebar footer with copyright, Heart, Valknar link, and GitFork source link
- Add author field to package.json

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

package.json

@@ -2,6 +2,10 @@
   "name": "kit-ui",
   "version": "0.1.0",
   "private": true,
+  "author": {
+    "name": "Valknar",
+    "email": "valknar@pivoine.art"
+  },
   "scripts": {
     "dev": "next dev --turbopack",
     "build": "next build",