feat(renderer): preview store with 300ms debounced source

tests/unit/stores/preview-store.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { usePreviewStore } from '@/stores/preview-store';
+
+describe('usePreviewStore', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    usePreviewStore.setState({ html: '', scrollRatio: 0, source: '' });
+  });
+
+  it('debounces source updates (300 ms)', () => {
+    usePreviewStore.getState().setSource('# a');
+    usePreviewStore.getState().setSource('# b');
+    usePreviewStore.getState().setSource('# c');
+    expect(usePreviewStore.getState().source).toBe('');
+    vi.advanceTimersByTime(300);
+    expect(usePreviewStore.getState().source).toBe('# c');
+  });
+
+  it('updates scroll ratio', () => {
+    usePreviewStore.getState().setScrollRatio(0.5);
+    expect(usePreviewStore.getState().scrollRatio).toBe(0.5);
+  });
+});