Alternative.disabled: announced but not yet takeable

The button renders disabled (title 'Not yet') and submit_answer
refuses the alternative server-side - lysbue's disabled flag, back as
content, for advertising a path before it works.

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

src/app.rs

@@ -1177,7 +1177,17 @@ fn AlternativeCard(
                                     children=move |e| view! { <p class="alt-encouragement">{e}</p> }
                                 />
                             </div>
-                            {if is_gateway {
+                            {if alternative.disabled {
+                                // Advertised, not yet takeable: the label
+                                // stays (it says what will be possible),
+                                // the button doesn't act.
+                                view! {
+                                    <button type="button" class="alt-submit soon" disabled=true title="Not yet">
+                                        {button_label.clone()}
+                                    </button>
+                                }
+                                .into_any()
+                            } else if is_gateway {
                                 let href = alternative.action.clone().unwrap_or_default();
                                 view! {
                                     <a class="alt-submit" href=href>{button_label.clone()}</a>
@@ -1860,6 +1870,9 @@ pub async fn submit_answer(
         .iter()
         .find(|a| a.name == alternative)
         .ok_or_else(|| ServerFnError::new("unknown alternative"))?;
+    if alt.disabled {
+        return Err(ServerFnError::new("this alternative isn't open yet"));
+    }
     let next = alt.action.clone();
     let record_as = alt.record_as.clone();