Render PDF headlines as document cards, not lightbox images

A starred PDF previously ran through the same crop-and-lightbox path
a photo does -- exactly the awkward treatment explicit headline
designation was meant to avoid. _headline_image.html.erb now branches
on @headline_asset.pdf?: a PDF renders as a linked card (a :medium
thumbnail, a file icon, its name), no gallery participation at all.
The existing image-headline and gallery-fallback logic is otherwise
unchanged, now scoped to an image-specific headline only.

Other attached PDFs -- headlined or not -- list below as plain links,
same reasoning as why non-headline photos still get a gallery
trigger: an attached document shouldn't go invisible just because
nothing's been starred yet.

headline_image's own render guard needed widening to cover a page
with PDFs attached but no images and no headline at all -- the one
case none of the existing conditions accounted for.

1 files changed, 2 insertions, 0 deletions

diff --git a/config/locales/en.yml b/config/locales/en.yml
index 1afcf219..9be01524 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -82,3 +82,5 @@ en:
     photo: "Photo %{name}"
     by: "by %{creator}"
     licensed_under: "Licensed under %{license}"
+
+  related_documents: "Related documents"