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.

.gitignore

@@ -4,5 +4,6 @@ venv/
 .venv/
 mock_output/
 web/static/*.png
+web/static/uploads/
 config.json
 config.json.tmp