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/lifecycle.rs

@@ -54,6 +54,7 @@ pub struct KernelStatusResponse {
 ///
 /// If kernel already exists with the same config, returns existing status.
 /// If config changed, reboots kernel with new config.
+// @connected
 #[tauri::command]
 pub async fn kernel_init(
     state: State<'_, KernelState>,
@@ -202,6 +203,7 @@ pub async fn kernel_init(
 }
 
 /// Get kernel status
+// @connected
 #[tauri::command]
 pub async fn kernel_status(
     state: State<'_, KernelState>,
@@ -227,6 +229,7 @@ pub async fn kernel_status(
 }
 
 /// Shutdown the kernel
+// @reserved: 暂无前端集成
 #[tauri::command]
 pub async fn kernel_shutdown(
     state: State<'_, KernelState>,
@@ -254,6 +257,7 @@ pub async fn kernel_shutdown(
 ///
 /// Writes relevant config values (agent, llm categories) to the TOML config file.
 /// The changes take effect on the next Kernel restart.
+// @connected
 #[tauri::command]
 pub async fn kernel_apply_saas_config(
     configs: Vec<SaasConfigItem>,