chore: replace Unicode/emoji icons with Lucide React

Adds lucide-react and replaces all HTML entity and emoji icons across
the UI with proper SVG icon components. Tree-shaken — only imported
icons are bundled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/package.json

@@ -16,6 +16,7 @@
   "dependencies": {
     "@cameleer/design-system": "^0.1.17",
     "@tanstack/react-query": "^5.90.21",
+    "lucide-react": "^1.7.0",
     "openapi-fetch": "^0.17.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",