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/content.rs

@@ -111,6 +111,11 @@ pub struct Alternative {
     pub description: String,
     #[serde(default)]
     pub action: Option<String>,
+    /// Shown but not takeable yet - the button renders disabled and the
+    /// server refuses the submission. For advertising a path before it
+    /// works (lysbue had the same flag).
+    #[serde(default)]
+    pub disabled: bool,
     #[serde(default)]
     pub consequence: Vec<String>,
     #[serde(default)]