Report page: full-width layout, attachment previews, screenshot lightbox ...

- stretch the kit's GnTabs (.tabs-panels shrink-wraps as a flex item) so the
  details grid and panels span the whole shell
- attachments tab: image thumbnails with filename/size tiles instead of a
  bare file list
- ScreenshotViewer: fullscreen lightbox with wheel/toolbar zoom capped at the
  image's natural resolution, drag-to-pan, Esc/backdrop close (a drag that
  ends off the image no longer closes it)

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