Implement User Growth Graph

**Description:**
Add a graph displaying user growth over time.

**Data:**

- Last 30 days
- Evolution curve

**Checklist:**

- [ ] Choose chart library (Recharts recommended)
- [ ] Connect to backend
- [ ] Display empty state
- [ ] Responsive chart

No files, tests, entry points, or backend source are named. Start by locating the admin frontend’s user-data entry point and existing chart patterns, then confirm the data source before choosing a chart library; done means a responsive last-30-days growth graph with an evolution curve and an empty state.

Written by the indexing model from the issue text.