feat(home): add AI story expand button and modal

tests/integration/api/contract/llm-observe-routes.test.ts

@@ -171,6 +171,13 @@ const ROUTE_CASES: ReadonlyArray<LLMRouteCase> = [
     expectedTargetType: 'NovelPromotionLocationDesign',
     expectedProjectId: 'project-1',
   },
+  {
+    routeFile: 'src/app/api/user/ai-story-expand/route.ts',
+    body: { prompt: '宫廷复仇女主回京' },
+    expectedTaskType: TASK_TYPE.AI_STORY_EXPAND,
+    expectedTargetType: 'HomeAiStoryExpand',
+    expectedProjectId: 'home-ai-write',
+  },
   {
     routeFile: 'src/app/api/novel-promotion/[projectId]/ai-modify-appearance/route.ts',
     body: {
@@ -336,7 +343,7 @@ describe('api contract - llm observe routes (behavior)', () => {
   })
 
   it('keeps expected coverage size', () => {
-    expect(ROUTE_CASES.length).toBe(22)
+    expect(ROUTE_CASES.length).toBe(23)
   })
 
   for (const routeCase of ROUTE_CASES) {