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-chat.ts

@@ -109,7 +109,7 @@ export function installChatMethods(ClientClass: { prototype: KernelClient }): vo
             }
             break;
 
-          case 'tool_start':
+          case 'toolStart':
             log.debug('Tool started:', streamEvent.name, streamEvent.input);
             if (callbacks.onTool) {
               callbacks.onTool(
@@ -120,7 +120,7 @@ export function installChatMethods(ClientClass: { prototype: KernelClient }): vo
             }
             break;
 
-          case 'tool_end':
+          case 'toolEnd':
             log.debug('Tool ended:', streamEvent.name, streamEvent.output);
             if (callbacks.onTool) {
               callbacks.onTool(
@@ -145,7 +145,7 @@ export function installChatMethods(ClientClass: { prototype: KernelClient }): vo
             }
             break;
 
-          case 'iteration_start':
+          case 'iterationStart':
             log.debug('Iteration started:', streamEvent.iteration, '/', streamEvent.maxIterations);
             // Don't need to notify user about iterations
             break;
@@ -201,10 +201,17 @@ export function installChatMethods(ClientClass: { prototype: KernelClient }): vo
   };
 
   /**
-   * Cancel a stream (no-op for internal kernel)
+   * Cancel an active stream by session ID.
+   * Invokes the Rust `cancel_stream` command which sets the AtomicBool flag
+   * checked by the spawned streaming task each iteration.
    */
-  proto.cancelStream = function (this: KernelClient, _runId: string): void {
-    // No-op: internal kernel doesn't support stream cancellation
+  proto.cancelStream = async function (this: KernelClient, sessionId: string): Promise<void> {
+    try {
+      await invoke('cancel_stream', { sessionId });
+      log.debug('Cancel stream requested for session:', sessionId);
+    } catch (err) {
+      log.warn('Failed to cancel stream:', err);
+    }
   };
 
   // ─── Default Agent ───