A slideshow across a page: pictures the full width of the screen, captions over them

slideshow on a page: max_height in svh, slides with an image (a path
in the content repo or https), a caption laid over the picture's foot,
and optionally the picture's size so its place is held before it
loads. Swiper upgrades it; before that it is a row that scrolls.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GT37Z1Xtfd9pUuQtMTg6Yt

src/content/validate.rs

@@ -146,6 +146,25 @@ pub fn validate_questions(
         }
     }
 
+    // A slideshow shows something, at a height that leaves the page
+    // its question: between a fifth and the whole of the screen.
+    for question in questions.values() {
+        if let Some(show) = &question.slideshow {
+            if show.slides.is_empty() {
+                anyhow::bail!("question {:?}: slideshow has no slides", question.id);
+            }
+            if !(20..=100).contains(&show.max_height) {
+                anyhow::bail!("question {:?}: slideshow max_height is {} svh; between 20 and 100", question.id, show.max_height);
+            }
+            for slide in &show.slides {
+                let ok = slide.image.starts_with("https://") || crate::content::is_safe_site_path(slide.image.trim_start_matches("/site/").trim_start_matches('/'));
+                if !ok {
+                    anyhow::bail!("question {:?}: slideshow image {:?} is neither an https address nor a path in the content repo", question.id, slide.image);
+                }
+            }
+        }
+    }
+
     // A mail to the record this one belongs to goes through a field of
     // the form: `{notice.email}` needs a `type: record` field called
     // `notice` on every form that records into the bucket, or the mail