Hidden fields: carrier value without a label row

A type: hidden requirement rendered through the fallback branch, whose
label wrapper shows the field name — so a preset key listed as a second
visible field under the email. Bare hidden input, no row.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y42TyF8Zu7NGRR2893vNcZ

src/app.rs

@@ -1244,6 +1244,20 @@ fn AlternativeCard(
                                                     .into_any();
                                                 }
 
+                                                if req.kind == "hidden" {
+                                                    // A carrier value only — no label row, or a
+                                                    // preset key reads as a second visible field.
+                                                    return view! {
+                                                        <input
+                                                            id=field_id
+                                                            type="hidden"
+                                                            prop:value=move || sig.get()
+                                                            on:input=move |ev| sig.set(event_target_value(&ev))
+                                                        />
+                                                    }
+                                                    .into_any();
+                                                }
+
                                                 view! {
                                                     <label class="field" for=label_for>
                                                         {label_text}