fix(layout): auto-réparer l'onglet actif

stale

Le backend renvoie l'id actif autoritaire et le
    frontend l'adopte pour éviter de rejouer un layout
    disparu après overwrite externe.

Co-Authored-By: Claude Opus 4.8
    <noreply@anthropic.com>

crates/app-tauri/src/dto.rs

@@ -390,7 +390,7 @@ pub fn parse_session_id(raw: &str) -> Result<SessionId, ErrorDto> {
 
 use application::{
     CreateLayoutOutput, DeleteLayoutOutput, LayoutInfo, LayoutOperation, ListLayoutsOutput,
-    LoadLayoutOutput, MutateLayoutOutput,
+    LoadLayoutOutput, MutateLayoutOutput, SetActiveLayoutOutput,
 };
 use domain::{AgentId, Direction, LayoutId, LayoutTree, NodeId};
 
@@ -700,6 +700,23 @@ pub struct DeleteLayoutRequestDto {
     pub layout_id: String,
 }
 
+/// Response DTO for `set_active_layout`.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SetActiveLayoutResultDto {
+    /// The layout actually activated — equals the requested id when valid, else
+    /// the unchanged current active id (self-healing fallback). Authoritative.
+    pub active_id: String,
+}
+
+impl From<SetActiveLayoutOutput> for SetActiveLayoutResultDto {
+    fn from(out: SetActiveLayoutOutput) -> Self {
+        Self {
+            active_id: out.active_id.to_string(),
+        }
+    }
+}
+
 /// Request DTO for `set_active_layout`.
 #[derive(Debug, Clone, Deserialize)]
 #[serde(rename_all = "camelCase")]