feat(layout): supporte la création de layouts plugins (customPluginLayout)

Étend le flux de création de layout backend (DTO, usecases, store) et le
frontend (sélecteur, adaptateurs, LayoutTabs/LayoutGrid) pour permettre
d'ouvrir un layout déclaré par un plugin installé (ex. Android Health),
sans passer par le message bloquant "extension backend pas encore livrée".

Ticket #141 — QA vert.

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

crates/domain/src/layout.rs

@@ -603,6 +603,38 @@ impl LayoutTree {
         Ok(tree)
     }
 
+    /// Replaces the opaque state of a plugin-provided custom layout leaf.
+    ///
+    /// Pure: returns a new validated tree.
+    ///
+    /// # Errors
+    /// - [`LayoutError::NodeNotFound`] if `target` is not a custom plugin layout
+    ///   leaf in the tree.
+    pub fn set_plugin_layout_state(
+        &self,
+        target: NodeId,
+        state: serde_json::Value,
+    ) -> Result<Self, LayoutError> {
+        let mut found = false;
+        let root = map_node(&self.root, &mut |node| {
+            if let LayoutNode::CustomPluginLayout(cell) = node {
+                if cell.id == target {
+                    found = true;
+                    let mut cell = cell.clone();
+                    cell.state = state.clone();
+                    return LayoutNode::CustomPluginLayout(cell);
+                }
+            }
+            node.clone()
+        });
+        if !found {
+            return Err(LayoutError::NodeNotFound(target));
+        }
+        let tree = Self { root };
+        tree.validate()?;
+        Ok(tree)
+    }
+
     /// Sets (or, with `None`, clears) the **engine session id** cache
     /// ([`LeafCell::engine_session_id`]) on the leaf `target` — the resumable id of
     /// the current provider (id Claude `--resume`, UUID minté pour `--session-id`).