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/index.ts

@@ -66,6 +66,14 @@ export type {
   GenerationProgressEvent,
 } from './classroomStore';
 
+// === Chat Stores ===
+export { useChatStore } from './chatStore';
+export type { Message, MessageFile, CodeBlock } from './chatStore';
+export { useConversationStore } from './chat/conversationStore';
+export { useMessageStore } from './chat/messageStore';
+export { useStreamStore } from './chat/streamStore';
+export { useArtifactStore } from './chat/artifactStore';
+
 // === Store Initialization ===
 
 import { getClient } from './connectionStore';