Phase 3: Redesign homepage + natural Thai text

- Add use cases section with 4 scenario cards (news, policy, business, fiction)
- Rewrite Thai home section text to be natural (not translated from Chinese)
- Add use case locale keys for th/en/zh
- Add CSS for use cases grid with hover effects

locales/zh.json

@@ -69,7 +69,16 @@
     "promptPlaceholder": "// 用自然语言输入模拟或预测需求（例.武大若发布撤销肖某处分的公告，会引发什么舆情走向）",
     "engineBadge": "引擎: CrowdSight-V1.0",
     "startEngine": "启动引擎",
-    "initializing": "初始化中..."
+    "initializing": "初始化中...",
+    "useCases": "可以做什么",
+    "useCase1Title": "模拟新闻与事件",
+    "useCase1Desc": "上传新闻 → 模拟公众在社交媒体上的反应",
+    "useCase2Title": "模拟政策与法规",
+    "useCase2Desc": "上传政策草案 → 模拟对不同群体的影响",
+    "useCase3Title": "模拟商业与广告",
+    "useCase3Desc": "上传广告计划 → 模拟传播和受众反馈",
+    "useCase4Title": "模拟小说与故事",
+    "useCase4Desc": "上传故事 → 模拟角色行为并预测结局"
   },
   "main": {
     "layoutGraph": "图谱",