refactor(store): split gatewayStore into specialized domain stores

Major restructuring:
- Split monolithic gatewayStore into 5 focused stores:
  - connectionStore: WebSocket connection and gateway lifecycle
  - configStore: quickConfig, workspaceInfo, MCP services
  - agentStore: clones, usage stats, agent management
  - handStore: hands, approvals, triggers, hand runs
  - workflowStore: workflows, workflow runs, execution

- Update all components to use new stores with selector pattern
- Remove

desktop/src/components/Settings/UsageStats.tsx

@@ -1,9 +1,12 @@
 import { useEffect, useState } from 'react';
-import { useGatewayStore } from '../../store/gatewayStore';
+import { useAgentStore } from '../../store/agentStore';
+import { useConnectionStore } from '../../store/connectionStore';
 import { BarChart3, TrendingUp, Clock, Zap } from 'lucide-react';
 
 export function UsageStats() {
-  const { usageStats, loadUsageStats, connectionState } = useGatewayStore();
+  const usageStats = useAgentStore((s) => s.usageStats);
+  const loadUsageStats = useAgentStore((s) => s.loadUsageStats);
+  const connectionState = useConnectionStore((s) => s.connectionState);
   const [timeRange, setTimeRange] = useState<'7d' | '30d' | 'all'>('7d');
 
   useEffect(() => {