Feature.link: a card's name may point elsewhere

An https URL on a feature renders its name as an outbound link -
an announcement's programme page, a venue. Validated on load.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/content.rs

@@ -237,6 +237,11 @@ pub struct Feature {
     /// rendered via Iconify's public SVG API - no icon library bundled.
     #[serde(default)]
     pub icon: Option<String>,
+    /// An https URL the card's name links to - an announcement's
+    /// programme page, a venue. Plain-text descriptions can't carry
+    /// links, so this is the one place a card points elsewhere.
+    #[serde(default)]
+    pub link: Option<String>,
     #[serde(default)]
     pub requirements: Vec<Requirement>,
     /// Live data this feature pulls in. Read-only unless `transitions`
@@ -1017,6 +1022,16 @@ pub fn validate_questions(
             }
         }
         for alternative in &question.alternatives {
+            for feature in &alternative.features {
+                if let Some(link) = &feature.link {
+                    if !link.starts_with("https://") {
+                        anyhow::bail!(
+                            "question {:?} feature {:?}: link {:?} must be an https:// URL",
+                            question.id, feature.name, link
+                        );
+                    }
+                }
+            }
             // A dangling action is a literal dead end: the submit
             // button navigates to "Nothing here".
             if let Some(action) = &alternative.action {