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-tauri/src/kernel_commands/approval.rs

@@ -25,6 +25,7 @@ pub struct ApprovalResponse {
 }
 
 /// List pending approvals
+// @reserved: 暂无前端集成
 #[tauri::command]
 pub async fn approval_list(
     state: State<'_, KernelState>,
@@ -48,6 +49,7 @@ pub async fn approval_list(
 /// When approved, the kernel's `respond_to_approval` internally spawns the Hand
 /// execution.  We additionally emit Tauri events so the frontend can track when
 /// the execution finishes, since the kernel layer has no access to the AppHandle.
+// @connected
 #[tauri::command]
 pub async fn approval_respond(
     app: AppHandle,