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>

frontend/src/features/layout/layout.ts

@@ -282,6 +282,18 @@ export function applyOperation(
       if (!found) throw notFound(op.target);
       return { root };
     }
+    case "setPluginLayoutState": {
+      let found = false;
+      const root = mapNode(tree.root, (n) => {
+        if (n.type === "customPluginLayout" && n.node.id === op.target) {
+          found = true;
+          return { type: "customPluginLayout", node: { ...n.node, state: op.state } };
+        }
+        return n;
+      });
+      if (!found) throw notFound(op.target);
+      return { root };
+    }
   }
 }
 
@@ -324,15 +336,10 @@ export function droppedSessions(
 }
 
 /**
- * Locally patches a `customPluginLayout` node's opaque `state` (#43, F4).
- *
- * Client-side only: the backend has no `LayoutOperation` variant to persist
- * plugin layout state yet (carnet v2 §3.5 — no backend refonte expected for
- * F4), so this does NOT call `LayoutGateway.mutateLayout`. It's the same
- * "real, in-session, not yet cross-restart-persisted" contract every plugin
- * component's `setState` gets: the tree re-renders with the new state
- * immediately, but a reload re-fetches the last **persisted** value from the
- * backend. Returns `tree` unchanged if no such node is found.
+ * Pure helper for patching a `customPluginLayout` node's opaque `state`.
+ * The runtime path persists the same change through `mutate_layout` with
+ * `setPluginLayoutState`; this helper remains useful for local tree transforms
+ * and focused parsing tests.
  */
 export function setCustomPluginLayoutState(
   tree: LayoutTree,