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/application/src/layout/management.rs

@@ -297,6 +297,15 @@ pub struct SetActiveLayoutInput {
     pub layout_id: LayoutId,
 }
 
+/// Output of [`SetActiveLayout::execute`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SetActiveLayoutOutput {
+    /// The id of the layout that was **actually** made active. Equals the
+    /// requested id when it exists; otherwise the unchanged current active id
+    /// (self-healing fallback). Authoritative for the frontend (invariant I4).
+    pub active_id: LayoutId,
+}
+
 /// Switches the active layout of a project.
 pub struct SetActiveLayout {
     store: Arc<dyn ProjectStore>,
@@ -317,20 +326,30 @@ impl SetActiveLayout {
 
     /// Sets the active layout.
     ///
+    /// A stale requested id (e.g. an `activeId` left over after git overwrote
+    /// `layouts.json`) must **not** freeze the workspace: instead of hard-
+    /// erroring, it degrades silently to the current active layout (invariant
+    /// I3). The returned [`SetActiveLayoutOutput::active_id`] is the id that was
+    /// actually activated and is authoritative for the frontend (invariant I4).
+    ///
     /// # Errors
-    /// [`AppError::NotFound`] if the project or layout is unknown.
-    pub async fn execute(&self, input: SetActiveLayoutInput) -> Result<(), AppError> {
+    /// - [`AppError::FileSystem`] on persistence failure,
+    /// - [`AppError::Store`] on registry I/O failure.
+    pub async fn execute(
+        &self,
+        input: SetActiveLayoutInput,
+    ) -> Result<SetActiveLayoutOutput, AppError> {
         let project = self.store.load_project(input.project_id).await?;
         let mut doc = resolve_doc(self.fs.as_ref(), &project).await?;
-        if doc.find(input.layout_id).is_none() {
-            return Err(AppError::NotFound(format!("layout {}", input.layout_id)));
-        }
-        doc.active_id = input.layout_id;
+        // Self-heal: keep the requested id when valid, else fall back to the
+        // (always-valid, I2) current active id rather than erroring.
+        let active_id = doc.resolve_existing_id(Some(input.layout_id));
+        doc.active_id = active_id;
 
         persist_doc(self.fs.as_ref(), &project, &doc).await?;
         self.events.publish(DomainEvent::LayoutChanged {
             project_id: input.project_id,
         });
-        Ok(())
+        Ok(SetActiveLayoutOutput { active_id })
     }
 }