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/static/frame_layout.js

@@ -286,11 +286,11 @@ window.addEventListener('resize', () => {
 // icon was clicked).
 const DIALOG_INIT = {
   photos: initPhotosDialog, calendar: initCalendarDialog, whiteboard: initWhiteboardDialog,
-  tasks: initTasksDialog,
+  tasks: initTasksDialog, static: initStaticDialog,
 };
 const DIALOG_CLOSE = {
   photos: closePhotosDialog, calendar: closeCalendarDialog, whiteboard: closeWhiteboardDialog,
-  tasks: closeTasksDialog,
+  tasks: closeTasksDialog, static: closeStaticDialog,
 };
 
 let openDialogWidgetType = null;