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>

src/index.css

@@ -997,3 +997,22 @@ input[type="text"]:disabled {
   transform: translateY(-2px);
   box-shadow: 0 6px 16px rgba(85, 104, 211, 0.4);
 }
+
+/* Topic cards grid for discovery */
+.topic-card-hover {
+  padding: 25px;
+  background: white;
+  border-radius: 12px;
+  border: 2px solid #e0e0e0;
+  text-align: center;
+  cursor: pointer;
+  transition: all 0.3s ease;
+  box-shadow: 0 2px 4px rgba(0,0,0,0.05);
+}
+
+.topic-card-hover:hover {
+  border-color: #667eea;
+  background: #fafbfc;
+  transform: translateY(-3px);
+  box-shadow: 0 6px 16px rgba(102, 126, 234, 0.2);
+}