Landscape orientation, redesigned web UI, rotation on/off + minutes, weather by city name

- Panel is mounted sideways: render landscape (600x400), rotate to the
  panel's native portrait orientation right before writing to hardware.
- Web UI: card-based layout, dark mode, active nav highlighting, save
  confirmation toasts, error banners.
- Rotation: separate enabled/disabled toggle (off pins to the first screen
  in the list) and the interval is now in minutes instead of raw seconds.
  Migrates existing config.json from the old interval_seconds field.
- Weather: type a city name instead of hunting down coordinates, resolved
  via Open-Meteo's free geocoding API. Manual lat/long still available as
  a fallback.

README.md

@@ -45,8 +45,8 @@ can't drive the same panel at once -- but nothing is copied from or reused
 out of it. Safe to re-run.
 
 Then open `http://<pi-ip>:8080`, fill in Navidrome credentials under
-**Navidrome** and coordinates under **Weather**. Everything saves
-immediately and takes effect within a few seconds -- no restart needed.
+**Navidrome** and a city under **Weather**. Everything saves immediately and
+takes effect within a few seconds -- no restart needed.
 
 ## Updating