dashboard: adopt SolarAssistant layout and CSS, eliminate flicker on updates

- Rebuild the dashboard around the real SolarAssistant HTML/CSS structure
  from 192.168.12.12:8080 (app.css and status icons saved locally).
- Header stays consistent across all pages: Dashboard, Charts, Historical,
  Automation, Configuration — each with icon + caption.
- Main page uses SA card-grid dashboard: status rows (Inverter, Solar,
  Grid, Battery), gauges (Load, Solar, Grid, Battery), and charts
  (PV Today, 24h Overview, Battery Power, Battery SOC).
- Clicking a status row or gauge drills into the subsystem detail page.
- No-flicker rendering: values and images are only updated when new data
  arrives; battery icon is preloaded before swapping.
- Charts tab: 30-day per-subsystem charts.
- Historical tab: 1-year charts + daily totals.
- Automation/Configuration tabs use the same header and card styling.

Author: Jeremy Ewing