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/store/chat/messageStore.ts

@@ -0,0 +1,98 @@
+/**
+ * MessageStore — manages token tracking and subtask mutations.
+ *
+ * Extracted from chatStore.ts as part of the structured refactor (Phase 3).
+ *
+ * Design note: The `messages[]` array stays in chatStore because
+ * `sendMessage` and `initStreamListener` use Zustand's `set((s) => ...)`
+ * pattern for high-frequency streaming deltas (dozens of updates per second).
+ * Moving messages out would force every streaming callback through
+ * `getState().updateMessage()` — adding overhead and breaking the
+ * producer-writes, consumer-reads separation that Zustand excels at.
+ *
+ * This store owns:
+ *   - Token usage counters (totalInputTokens, totalOutputTokens)
+ *   - Subtask mutation helpers (addSubtask, updateSubtask)
+ *
+ * Messages are read from chatStore by consumers that need them.
+ *
+ * @see docs/superpowers/specs/2026-04-02-chatstore-refactor-design.md §3.3
+ */
+
+import { create } from 'zustand';
+import type { Subtask } from '../../components/ai';
+
+// ---------------------------------------------------------------------------
+// State interface
+// ---------------------------------------------------------------------------
+
+export interface MessageState {
+  totalInputTokens: number;
+  totalOutputTokens: number;
+
+  // Token tracking
+  addTokenUsage: (inputTokens: number, outputTokens: number) => void;
+  getTotalTokens: () => { input: number; output: number; total: number };
+  resetTokenUsage: () => void;
+
+  // Subtask mutations (delegated to chatStore internally)
+  addSubtask: (messageId: string, task: Subtask) => void;
+  updateSubtask: (messageId: string, taskId: string, updates: Partial<Subtask>) => void;
+}
+
+// ---------------------------------------------------------------------------
+// Internal reference to chatStore for message mutations
+// ---------------------------------------------------------------------------
+let _chatStore: {
+  getState: () => {
+    addSubtask: (messageId: string, task: Subtask) => void;
+    updateSubtask: (messageId: string, taskId: string, updates: Partial<Subtask>) => void;
+  };
+} | null = null;
+
+/**
+ * Inject chatStore reference for subtask delegation.
+ * Called by chatStore during initialization to avoid circular imports.
+ */
+export function setMessageStoreChatStore(store: typeof _chatStore): void {
+  _chatStore = store;
+}
+
+// ---------------------------------------------------------------------------
+// Store
+// ---------------------------------------------------------------------------
+
+export const useMessageStore = create<MessageState>()((set, get) => ({
+  totalInputTokens: 0,
+  totalOutputTokens: 0,
+
+  addTokenUsage: (inputTokens: number, outputTokens: number) =>
+    set((state) => ({
+      totalInputTokens: state.totalInputTokens + inputTokens,
+      totalOutputTokens: state.totalOutputTokens + outputTokens,
+    })),
+
+  getTotalTokens: () => {
+    const { totalInputTokens, totalOutputTokens } = get();
+    return {
+      input: totalInputTokens,
+      output: totalOutputTokens,
+      total: totalInputTokens + totalOutputTokens,
+    };
+  },
+
+  resetTokenUsage: () =>
+    set({ totalInputTokens: 0, totalOutputTokens: 0 }),
+
+  addSubtask: (messageId: string, task: Subtask) => {
+    if (_chatStore) {
+      _chatStore.getState().addSubtask(messageId, task);
+    }
+  },
+
+  updateSubtask: (messageId: string, taskId: string, updates: Partial<Subtask>) => {
+    if (_chatStore) {
+      _chatStore.getState().updateSubtask(messageId, taskId, updates);
+    }
+  },
+}));