perf: add skeleton loading and async data loading for dashboard

- Add skeleton screens for live status, statistics, and charts sections
- Load data in stages: live data first, then stats and charts in background
- Use lightweight ChartDataPoint DTO to reduce SignalR payload size
- Reduce max chart points from 500 to 150 per sensor
- Query only latest timestamp for live data instead of full hour
- Add separate loading states for each section

Author: SwaggyMacro