Templated actions; places are pickable in the gesture input

action: /shape/{curve.key} fills placeholders from the submitted
responses and must land on a dynamic page (validated). The gesture
widget now treats relay 'place' (kept) and 'echo' (live presence)
frames as pickable options: picking one re-derives the input's key -
that place's decode becomes the ghost under the stroke and the answer
records {key: picked, own_key, selected_from, selected_distance}, the
labelled pair that later ranks places and calibrates the key. Dedupe
by key; an empty state when nothing is kept at the shape.

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

src/app.rs

@@ -1999,11 +1999,18 @@ pub async fn submit_answer(
     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();
 
     let responses: serde_json::Value = serde_json::from_str(&responses_json)
         .map_err(|e| ServerFnError::new(format!("invalid responses: {e}")))?;
+    // `/shape/{curve.key}`: the next page may be chosen by the answer.
+    let next = match alt.action.as_deref() {
+        Some(action) => Some(
+            crate::content::template_action(action, &responses)
+                .ok_or_else(|| ServerFnError::new("this answer is missing what the next page needs"))?,
+        ),
+        None => None,
+    };
 
     let parent_hashes: Vec<String> = parent_hash.into_iter().collect();
     let timestamp_ms = chrono::Utc::now().timestamp_millis();