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.

README.md

@@ -1,9 +1,10 @@
 # Paperwall
 
 A standalone dashboard for one Waveshare `epd4in0e` (4", 6-color) e-ink panel:
-Navidrome now-playing, clock, and weather, switchable/controllable from a
-local web UI. Started as an InkyPi replacement, but doesn't depend on InkyPi
-at all -- own venv, own vendored copy of the Waveshare driver files.
+Navidrome now-playing, clock, weather, and pictures (upload or URL),
+switchable/controllable from a local web UI. Started as an InkyPi
+replacement, but doesn't depend on InkyPi at all -- own venv, own vendored
+copy of the Waveshare driver files.
 
 Confirmed by reading the vendor driver directly: this panel has no
 partial-refresh mode at all -- every update is a full flash. So the design