v0.50.0: m Recolour local radar rain and add 25/50/100 km map zoom

- KNMI draws light rain white and grey, which vanished on the light
  basemap; local radar now uses a sky blue to magenta scale in both themes.
- The web local radar zooms between 25, 50 and 100 km; the widget stays
  at 50 km. `weather radar --radius` exports the same maps.
- Every KNMI request covers 100 km at 512×512 and each zoom crops its
  centre, so zooming adds no KNMI requests.
- The 100 km background uses zoom 8 PDOK tiles because zoom 7 leaves the
  sea transparent.
- The local "+1h" button shows the real end of playback, e.g. "+2h".