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/a2a.rs

@@ -12,6 +12,7 @@ use super::KernelState;
 
 #[cfg(feature = "multi-agent")]
 /// Send a direct A2A message from one agent to another
+// @connected
 #[tauri::command]
 pub async fn agent_a2a_send(
     state: State<'_, KernelState>,
@@ -44,6 +45,7 @@ pub async fn agent_a2a_send(
 
 /// Broadcast a message from one agent to all other agents
 #[cfg(feature = "multi-agent")]
+// @connected
 #[tauri::command]
 pub async fn agent_a2a_broadcast(
     state: State<'_, KernelState>,
@@ -65,6 +67,7 @@ pub async fn agent_a2a_broadcast(
 
 /// Discover agents with a specific capability
 #[cfg(feature = "multi-agent")]
+// @connected
 #[tauri::command]
 pub async fn agent_a2a_discover(
     state: State<'_, KernelState>,
@@ -86,6 +89,7 @@ pub async fn agent_a2a_discover(
 
 /// Delegate a task to another agent and wait for response
 #[cfg(feature = "multi-agent")]
+// @connected
 #[tauri::command]
 pub async fn agent_a2a_delegate_task(
     state: State<'_, KernelState>,