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.

web/templates/base.html

@@ -198,6 +198,7 @@
       <a href="{{ url_for('index') }}" class="{{ 'active' if request.endpoint == 'index' }}">Status</a>
       <a href="{{ url_for('settings_navidrome') }}" class="{{ 'active' if request.endpoint == 'settings_navidrome' }}">Navidrome</a>
       <a href="{{ url_for('settings_weather') }}" class="{{ 'active' if request.endpoint == 'settings_weather' }}">Weather</a>
+      <a href="{{ url_for('settings_picture') }}" class="{{ 'active' if request.endpoint == 'settings_picture' }}">Picture</a>
       <a href="{{ url_for('settings_rotation') }}" class="{{ 'active' if request.endpoint == 'settings_rotation' }}">Rotation</a>
     </nav>
     {% if request.args.get('saved') %}