Replace preset topics with dynamic topic listing from server

Changed discover page to fetch and display real topics from the API:
- Added fetchTopics() to call client.offers.getTopics()
- Display actual topic names and active peer counts
- Added loading, error, and empty states
- Added refresh button to reload topics
- Improved UX with better error handling

Updated to @xtr-dev/rondevu-client@0.7.4

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -10,7 +10,7 @@
     "deploy": "npm run build && npx wrangler pages deploy dist --project-name=rondevu-demo"
   },
   "dependencies": {
-    "@xtr-dev/rondevu-client": "^0.7.3",
+    "@xtr-dev/rondevu-client": "^0.7.4",
     "@zxing/library": "^0.21.3",
     "qrcode": "^1.5.4",
     "react": "^18.2.0",