Add a static image widget (PNG/JPEG/GIF/BMP/WEBP/TIFF/PDF upload)

A new widget type showing a single user-uploaded image with no live
upstream to poll -- decoded once at upload time (PDF's first page via
pypdfium2, BSD-3-Clause/Apache-2.0, no copyleft exposure) into plain RGB
PNG bytes, then composed per a crop/stretch/letterbox display mode like
the photos widget.

server/app/grid.py

@@ -30,6 +30,7 @@ MIN_FOOTPRINT: dict[str, tuple[int, int]] = {
     "calendar": (3, 2),
     "whiteboard": (2, 2),
     "tasks": (2, 2),
+    "static": (1, 1),
 }
 
 Rect = tuple[int, int, int, int]  # (x, y, w, h)