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/hand.rs

@@ -110,6 +110,7 @@ impl From<zclaw_hands::HandResult> for HandResult {
 ///
 /// Returns hands from the Kernel's HandRegistry.
 /// Hands are registered during kernel initialization.
+// @connected
 #[tauri::command]
 pub async fn hand_list(
     state: State<'_, KernelState>,
@@ -128,6 +129,7 @@ pub async fn hand_list(
 /// Executes a hand with the given ID and input.
 /// If the hand has `needs_approval = true`, creates a pending approval instead.
 /// Returns the hand result as JSON, or a pending status with approval ID.
+// @connected
 #[tauri::command]
 pub async fn hand_execute(
     state: State<'_, KernelState>,
@@ -190,6 +192,7 @@ pub async fn hand_execute(
 /// 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.
+// @connected
 #[tauri::command]
 pub async fn hand_approve(
     app: AppHandle,
@@ -292,6 +295,7 @@ pub async fn hand_approve(
 }
 
 /// Cancel a hand execution
+// @connected
 #[tauri::command]
 pub async fn hand_cancel(
     state: State<'_, KernelState>,
@@ -330,6 +334,7 @@ pub async fn hand_cancel(
 // ============================================================
 
 /// Get detailed info for a single hand
+// @connected
 #[tauri::command]
 pub async fn hand_get(
     state: State<'_, KernelState>,
@@ -348,6 +353,7 @@ pub async fn hand_get(
 }
 
 /// Get status of a specific hand run
+// @connected
 #[tauri::command]
 pub async fn hand_run_status(
     state: State<'_, KernelState>,
@@ -375,6 +381,7 @@ pub async fn hand_run_status(
 }
 
 /// List run history for a hand (or all hands)
+// @connected
 #[tauri::command]
 pub async fn hand_run_list(
     state: State<'_, KernelState>,
@@ -409,6 +416,7 @@ pub async fn hand_run_list(
 }
 
 /// Cancel a running hand execution
+// @reserved: 暂无前端集成
 #[tauri::command]
 pub async fn hand_run_cancel(
     state: State<'_, KernelState>,