refactor(desktop): ChatStore structured split + IDB persistence + stream cancel

Split monolithic chatStore.ts (908 lines) into 4 focused stores:
- chatStore.ts: facade layer, owns messages[], backward-compatible selectors
- conversationStore.ts: conversation CRUD, agent switching, IndexedDB persistence
- streamStore.ts: streaming orchestration, chat mode, suggestions
- messageStore.ts: token tracking

Key fixes from 3-round deep audit:
- C1: Fix Rust serde camelCase vs TS snake_case mismatch (toolStart/toolEnd/iterationStart)
- C2: Fix IDB async rehydration race with persist.hasHydrated() subscribe
- C3: Add sessionKey to partialize to survive page refresh
- H3: Fix IDB migration retry on failure (don't set migrated=true in catch)
- M3: Fix ToolCallStep deduplication (toolStart creates, toolEnd updates)
- M-NEW-2: Clear sessionKey on cancelStream

Also adds:
- Rust backend stream cancellation via AtomicBool + cancel_stream command
- IndexedDB storage adapter with one-time localStorage migration
- HMR cleanup for cross-store subscriptions

desktop/src/store/agentStore.ts

@@ -8,6 +8,7 @@ import { create } from 'zustand';
 import type { GatewayClient } from '../lib/gateway-client';
 import type { AgentTemplateFull } from '../lib/saas-client';
 import { useChatStore } from './chatStore';
+import { useConversationStore } from './chat/conversationStore';
 
 // === Types ===
 
@@ -251,7 +252,7 @@ export const useAgentStore = create<AgentStore>((set, get) => ({
 
   loadUsageStats: async () => {
     try {
-      const { conversations } = useChatStore.getState();
+      const { conversations } = useConversationStore.getState();
       const tokenData = useChatStore.getState().getTotalTokens();
 
       let totalMessages = 0;