Add server-side rendered setup UI accessible via `heatwave web`.
The dashboard is now re-rendered per request and includes a nav bar
linking to the new /setup page. Setup provides full CRUD for profiles,
rooms, devices, occupants, AC units (with room assignment), scenario
toggles, and forecast fetching — all via POST/redirect/GET forms.
- Add ShowNav field to DashboardData for conditional nav bar
- Extract fetchForecastForProfile() for reuse by web handler
- Create setup.html.tmpl with Tailwind-styled entity sections
- Create web_handlers.go with 15 route handlers and flash cookies
- Switch web.go from pre-rendered to per-request dashboard rendering
- Graceful dashboard fallback when no forecast data exists
