feat(state): localStorage player prefs for story and action display

src/state/__tests__/prefs.test.ts

@@ -0,0 +1,25 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { getPrefs, setPrefs } from '../prefs';
+
+describe('prefs', () => {
+  beforeEach(() => {
+    vi.stubGlobal('localStorage', {
+      store: {} as Record<string, string>,
+      getItem(key: string) {
+        return this.store[key] ?? null;
+      },
+      setItem(key: string, value: string) {
+        this.store[key] = value;
+      },
+    });
+  });
+
+  it('returns defaults when localStorage empty', () => {
+    expect(getPrefs()).toEqual({ storyOpenMode: 'auto', actionDetailMode: 'inline' });
+  });
+
+  it('round-trips updated prefs', () => {
+    setPrefs({ storyOpenMode: 'manual', actionDetailMode: 'hover' });
+    expect(getPrefs().storyOpenMode).toBe('manual');
+  });
+});