feat: UI revamp with sidebar nav and grouped library

Replace top Nav with Sidebar, add lib/group.js for library grouping,
tokenized color/alert styles, and tighter Settings/Canvas export layout.
Adds @tabler/icons-react. Verified production build (standalone) passes,
so the Docker image built from this tree matches the local app.

Co-authored-by: Claude <claude-code@anthropic.com>

package.json

@@ -9,6 +9,7 @@
     "start": "next start"
   },
   "dependencies": {
+    "@tabler/icons-react": "^3.44.0",
     "@tailwindcss/postcss": "^4.3.1",
     "next": "14.2.18",
     "postcss": "^8.5.15",