Show progress while loading reads the image sizes

Before decoding, loadImages reads every header to size the batch against
the free memory. That pass was serial, silent and could not be cancelled,
so the loading window sat at "0 of N". LoadProgress gains onSized, the
headers are read in parallel on the same cursor-driven workers as the
decodes, and a cancel stops the pass. The window's bar runs twice,
"Checking sizes" then "Decoding", and the CLI prints a sizing line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -52,6 +52,10 @@ commit that made it. The interface shows this file under Changelog in its menu.
   videos exactly as before.
 
 ### Changed
+- Loading images shows its first pass too: the progress window counts the
+  files as their sizes are checked ("Checking sizes: 3 of 40") before it
+  counts the decodes, the headers are read in parallel, and Cancel works
+  during that pass as well. The command line shows a `sizing` line likewise.
 - The video preview plays sooner: it renders every third frame first and
   plays that draft, each frame held for three, then fills in the frames
   between while it plays. The finished preview and the saved video are