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/lib/kernel-client.ts

@@ -404,7 +404,7 @@ export interface KernelClient {
   // Chat (kernel-chat.ts)
   chat(message: string, opts?: { sessionKey?: string; agentId?: string }): Promise<{ runId: string; sessionId?: string; response?: string }>;
   chatStream(message: string, callbacks: import('./kernel-types').StreamCallbacks, opts?: { sessionKey?: string; agentId?: string; thinking_enabled?: boolean; reasoning_effort?: string; plan_mode?: boolean }): Promise<{ runId: string }>;
-  cancelStream(runId: string): void;
+  cancelStream(sessionId: string): Promise<void>;
   fetchDefaultAgentId(): Promise<string | null>;
   setDefaultAgentId(agentId: string): void;
   getDefaultAgentId(): string;