- Add get_usage_trend() and get_amount_trend() functions for day-over-day comparison
- Display trend arrows (up/down/flat) with percentage change in stat_today_usage and stat_today_amount
- Add colored left borders to all stat cards for visual distinction:
- Blue: usage, total users
- Purple: amount, total orgs
- Green: active users, new users this month
- Cyan: concurrent users
- Red: errors
- Enhance visual hierarchy with accent colors matching SVG icons