Add live-preview busy indicator for slow renders (M6, §04/§10)

src/app/stores/PreviewStore.test.ts

@@ -0,0 +1,54 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { usePreviewStore } from './PreviewStore';
+
+const store = () => usePreviewStore.getState();
+
+afterEach(() => {
+  // Reset to a known clean state between tests so store leaks don't affect order.
+  store().setError(null);
+  store().setBusy(false);
+});
+
+describe('PreviewStore — error slice', () => {
+  it('starts with null error', () => {
+    expect(store().error).toBeNull();
+  });
+
+  it('setError stores the provided message', () => {
+    store().setError('Rendering error: something went wrong.');
+    expect(store().error).toBe('Rendering error: something went wrong.');
+  });
+
+  it('setError(null) clears the message', () => {
+    store().setError('an error');
+    store().setError(null);
+    expect(store().error).toBeNull();
+  });
+});
+
+describe('PreviewStore — busy slice', () => {
+  it('starts with busy=false', () => {
+    expect(store().busy).toBe(false);
+  });
+
+  it('setBusy(true) sets busy to true', () => {
+    store().setBusy(true);
+    expect(store().busy).toBe(true);
+  });
+
+  it('setBusy(false) clears busy', () => {
+    store().setBusy(true);
+    store().setBusy(false);
+    expect(store().busy).toBe(false);
+  });
+
+  it('busy and error are independent — setting one does not affect the other', () => {
+    store().setBusy(true);
+    store().setError('some error');
+    expect(store().busy).toBe(true);
+    expect(store().error).toBe('some error');
+
+    store().setBusy(false);
+    expect(store().error).toBe('some error'); // error unchanged by clearing busy
+  });
+});