feat(renderer): command store persists userBindings (Phase 6)

- add userBindings map (commandId -> combo string)
- setUserBinding / clearUserBinding / getUserBinding actions
- persist middleware with partialize: handlers are runtime-only,
  userBindings are serialized
- TDD: 7 persistence tests + existing 10 command-store tests still pass
- 162/162 total tests pass

tests/unit/stores/command-store-persist.test.ts

@@ -0,0 +1,59 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { useCommandStore } from '@/stores/command-store';
+
+describe('useCommandStore — user bindings persistence', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    useCommandStore.setState({
+      handlers: {},
+      userBindings: {},
+    });
+  });
+
+  it('starts with empty userBindings', () => {
+    expect(useCommandStore.getState().userBindings).toEqual({});
+  });
+
+  it('setUserBinding stores a combo for a command id', () => {
+    useCommandStore.getState().setUserBinding('file.save', 'mod+shift+s');
+    expect(useCommandStore.getState().userBindings['file.save']).toBe('mod+shift+s');
+  });
+
+  it('setUserBinding overwrites an existing binding for the same command', () => {
+    useCommandStore.getState().setUserBinding('file.save', 'mod+shift+s');
+    useCommandStore.getState().setUserBinding('file.save', 'mod+alt+s');
+    expect(useCommandStore.getState().userBindings['file.save']).toBe('mod+alt+s');
+  });
+
+  it('clearUserBinding removes a binding', () => {
+    useCommandStore.getState().setUserBinding('file.save', 'mod+shift+s');
+    useCommandStore.getState().clearUserBinding('file.save');
+    expect(useCommandStore.getState().userBindings['file.save']).toBeUndefined();
+  });
+
+  it('clearUserBinding on a missing id is a no-op', () => {
+    expect(() => useCommandStore.getState().clearUserBinding('does.not.exist')).not.toThrow();
+  });
+
+  it('getUserBinding returns the combo or undefined', () => {
+    useCommandStore.getState().setUserBinding('file.save', 'mod+shift+s');
+    expect(useCommandStore.getState().getUserBinding('file.save')).toBe('mod+shift+s');
+    expect(useCommandStore.getState().getUserBinding('does.not.exist')).toBeUndefined();
+  });
+
+  it('persists userBindings to localStorage and restores on next mount', async () => {
+    useCommandStore.getState().setUserBinding('file.save', 'mod+shift+s');
+    useCommandStore.getState().setUserBinding('view.toggleSidebar', 'mod+b');
+
+    // Force the persist middleware to flush
+    await useCommandStore.persist?.flush?.();
+
+    const stored = JSON.parse(localStorage.getItem('mc-command-store') ?? '{}');
+    expect(stored.state.userBindings).toEqual({
+      'file.save': 'mod+shift+s',
+      'view.toggleSidebar': 'mod+b',
+    });
+    // Handlers must NOT be persisted (functions aren't serializable)
+    expect(stored.state.handlers).toBeUndefined();
+  });
+});