feat(renderer): command store (action registry for menu/shortcut/toolbar)

- useCommandStore: register/unregister/registerMany/dispatch/get
- keyed by id (e.g. 'file.open', 'view.toggleSidebar')
- TDD: 10 unit tests covering register, dispatch, unregister, batch
- foundation for Phase 6 native menu + toolbar wiring

src/renderer/stores/command-store.ts

@@ -0,0 +1,35 @@
+import { create } from 'zustand';
+
+export type CommandId = string;
+
+export type CommandHandler = (args?: unknown) => void | Promise<void>;
+
+interface CommandState {
+  handlers: Record<string, CommandHandler>;
+  register: (id: CommandId, handler: CommandHandler) => void;
+  registerMany: (handlers: Record<string, CommandHandler>) => void;
+  unregister: (id: CommandId) => void;
+  dispatch: (id: CommandId, args?: unknown) => void;
+  get: (id: CommandId) => CommandHandler | undefined;
+}
+
+export const useCommandStore = create<CommandState>((set, get) => ({
+  handlers: {},
+  register: (id, handler) =>
+    set((s) => ({ handlers: { ...s.handlers, [id]: handler } })),
+  registerMany: (handlers) =>
+    set((s) => ({ handlers: { ...s.handlers, ...handlers } })),
+  unregister: (id) =>
+    set((s) => {
+      const next = { ...s.handlers };
+      delete next[id];
+      return { handlers: next };
+    }),
+  dispatch: (id, args) => {
+    const handler = get().handlers[id];
+    if (handler) {
+      void handler(args);
+    }
+  },
+  get: (id) => get().handlers[id],
+}));