test(images): concept comparison for the Images page

images-concept.spec.mjs compares the production Images page with the
immutable concept on the concept-shaped fixture: heading, toolbar, filter
row, caption, grid, the first six cards, continuation and panel box in
Grid, List and Masonry, at 1600x1100 and 390x844, dark and light, normal
and reduced motion; plus hover zoom, the viewer title, focus restore,
Info selection, search and Load more with its toast. mapImagesConcept
applies only recorded deviations (D3, D8 and D16 follow-up PRs, D9, D10,
D14, D15, D17, D29, D31, D32); removed row items flow again from the
concept's own boxes. A mutation check proves the comparison fails on a
.media-info padding change and on removed Inspect buttons.

Corrections the comparison demonstrated:
- the grid and continuation carry the concept's ids (image-grid,
  gallery-continuation);
- bounded rendering reserves the card footer at its fractional height:
  offsetHeight rounded a wrapped 67.5px row up, so at 390px every card
  was a pixel taller and the grid 6px taller than the concept.

Two mismatches wait for a ruling and are pinned on both sides: the
category order (concept prompt order, production alphabetical) and the
singular count line ("1 positive prompt").

tests/browser/workspace_concept_fixture.py

@@ -430,6 +430,8 @@ def seed_workspace_concept(
     api.invalidate_gallery_cache()
     return {
         "epoch": epoch,
+        # The newest file's mtime (epoch seconds), for the D17 caption date.
+        "newest_modified": now - 60,
         "prompts": prompts,
         "images": images,
         "runs": runs,