feat: show received frontmatter fields in the message detail

src/demo.rs

@@ -158,6 +158,30 @@ fn plant(dir: &Path) -> Result<(), String> {
         true,
     )?;
 
+    // one inbox mail with the sender's own frontmatter fields, so the
+    // detail view has a custom header block to show
+    {
+        let sender = identity(11);
+        let uri = Address::parse("alice@example.org")
+            .unwrap()
+            .uri(&sender.pk());
+        let text = build_frontmatter(
+            &[
+                ("Subject", "canary rollout report"),
+                ("Reply-To", &uri),
+                ("Priority", "high"),
+                ("X-Stage", "canary at 5%"),
+            ],
+            "The custom fields on this one are the sender's own — carrier shows them between the meta line and the body.",
+        );
+        let envelope = seal(&sender, &pk, text.as_bytes(), now() - 12 * 3600, true)
+            .map_err(|e| e.to_string())?;
+        let id = message_id(&envelope);
+        store
+            .store_inbox(&id, &envelope, now() - 12 * 3600, false, false)
+            .map_err(|e| e.to_string())?;
+    }
+
     // requests: senders not in the contact list
     deliver(
         &identity(31),