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/render_utils.py

@@ -8,7 +8,7 @@ text and full-color cover art without worrying about the palette themselves.
 
 from pathlib import Path
 
-from PIL import ImageFont
+from PIL import Image, ImageFont
 
 FONTS_DIR = Path(__file__).resolve().parent.parent / "fonts"
 
@@ -77,3 +77,26 @@ def wrap_to_lines(draw, text: str, font: ImageFont.FreeTypeFont, max_width: int,
 
     lines.append(current)
     return lines[:max_lines]
+
+
+def fit_cover(image: Image.Image, width: int, height: int) -> Image.Image:
+    """Resize+crop to fill exactly width x height, mirroring CSS's
+    object-fit: cover -- no letterboxing, but edges may be cropped off."""
+    src_w, src_h = image.size
+    scale = max(width / src_w, height / src_h)
+    scaled = image.resize((round(src_w * scale), round(src_h * scale)), Image.LANCZOS)
+    left = (scaled.width - width) // 2
+    top = (scaled.height - height) // 2
+    return scaled.crop((left, top, left + width, top + height))
+
+
+def fit_contain(image: Image.Image, width: int, height: int, background=(255, 255, 255)) -> Image.Image:
+    """Resize to fit entirely within width x height, centered on a solid
+    background -- mirrors CSS's object-fit: contain, full image always
+    visible, letterboxed instead of cropped."""
+    src_w, src_h = image.size
+    scale = min(width / src_w, height / src_h)
+    scaled = image.resize((round(src_w * scale), round(src_h * scale)), Image.LANCZOS)
+    canvas = Image.new("RGB", (width, height), background)
+    canvas.paste(scaled, ((width - scaled.width) // 2, (height - scaled.height) // 2))
+    return canvas