Add Picture screen: upload a file or point at a URL

New screen alongside clock/weather/navidrome. Upload goes through the web
UI (validated with PIL before accepting, stored under web/static/uploads/,
gitignored), or point it at a URL re-fetched every 30 minutes. Cover/contain
fit modes (new fit_cover/fit_contain helpers in render_utils.py, generalized
from the square-only cover-art cropping already used by the Navidrome
screen), plus the same contrast-boost/grayscale options that screen has.

screens/__init__.py

@@ -1,10 +1,12 @@
 from screens.clock import ClockScreen
 from screens.navidrome import NavidromeScreen
+from screens.picture import PictureScreen
 from screens.weather import WeatherScreen
 
 SCREEN_CLASSES = {
     ClockScreen.id: ClockScreen,
     WeatherScreen.id: WeatherScreen,
+    PictureScreen.id: PictureScreen,
     NavidromeScreen.id: NavidromeScreen,
 }