fix: report backlog sync errors in dashboard

public/index.html

@@ -130,7 +130,7 @@
           await apiRequest('/api/backlog-sync', {
             method: 'POST',
             body: JSON.stringify({ guildId: state.selectedTextGuild, channelId: state.selectedTextChannel }),
-          }).catch((error) => logger.warn('Backlog sync failed:', error.message));
+          }).catch((error) => showError(`Backlog sync failed: ${error.message}`));
         }
         await fetchText().catch((error) => showError(error.message));
       }