Add Policy “Explain” API Endpoint

## 🟢 #5 Feature: Add “Live / Simulated” Status Badge (Header)

### 🧠 Context
Users cannot distinguish data source.

### 🎯 Goal
Display system mode clearly.

### 🛠️ Tasks
- [ ] Add header badge component
- [ ] States:
  - Live
  - Simulated
- [ ] Bind to system state

### ✅ Acceptance Criteria
- Badge updates dynamically
- Clear visual distinction

### 🏷️ Labels
good-first-issue, frontend