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/en.json

@@ -69,7 +69,16 @@
     "promptPlaceholder": "// Describe your simulation or prediction requirement in natural language",
     "engineBadge": "Engine: CrowdSight-V1.0",
     "startEngine": "Start Engine",
-    "initializing": "Initializing..."
+    "initializing": "Initializing...",
+    "useCases": "What you can do",
+    "useCase1Title": "Simulate news & events",
+    "useCase1Desc": "Upload news → Simulate public reactions on social media",
+    "useCase2Title": "Simulate policies & regulations",
+    "useCase2Desc": "Upload draft policy → Simulate impact on different groups",
+    "useCase3Title": "Simulate business & advertising",
+    "useCase3Desc": "Upload ad plan → Simulate spread and audience response",
+    "useCase4Title": "Simulate fiction & stories",
+    "useCase4Desc": "Upload story → Simulate character behavior and predict endings"
   },
   "main": {
     "layoutGraph": "Graph",