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/adapters/mock/index.ts

@@ -27,6 +27,7 @@ import type {
   LayoutList,
   LayoutOperation,
   LayoutTree,
+  PluginLayoutOrigin,
   LocalModelServerConfig,
   ModelServerCommandPreview,
   Memory,
@@ -958,6 +959,7 @@ interface MockLayoutEntry {
   id: string;
   name: string;
   kind: LayoutKind;
+  pluginOrigin?: PluginLayoutOrigin | null;
   tree: LayoutTree;
 }
 
@@ -1028,14 +1030,38 @@ export class MockLayoutGateway implements LayoutGateway {
 
   async listLayouts(projectId: string): Promise<LayoutList> {
     const ps = this.getProjectLayouts(projectId);
-    const layouts: LayoutInfo[] = ps.layouts.map((l) => ({ id: l.id, name: l.name, kind: l.kind }));
+    const layouts: LayoutInfo[] = ps.layouts.map((l) => ({
+      id: l.id,
+      name: l.name,
+      kind: l.kind,
+      pluginOrigin: l.pluginOrigin,
+    }));
     return { layouts, activeId: ps.activeId };
   }
 
-  async createLayout(projectId: string, name: string, kind: LayoutKind = "terminal"): Promise<{ layoutId: string }> {
+  async createLayout(
+    projectId: string,
+    name: string,
+    kind: LayoutKind = "terminal",
+    pluginOrigin?: PluginLayoutOrigin,
+  ): Promise<{ layoutId: string }> {
     const ps = this.getProjectLayouts(projectId);
     const layoutId = `layout-${Math.random().toString(36).slice(2, 10)}`;
-    ps.layouts.push({ id: layoutId, name, kind, tree: singleLeafTree() });
+    const tree =
+      kind === "plugin" && pluginOrigin
+        ? {
+            root: {
+              type: "customPluginLayout" as const,
+              node: {
+                id: `plugin-cell-${Math.random().toString(36).slice(2, 10)}`,
+                pluginId: pluginOrigin.pluginId,
+                layoutType: pluginOrigin.layoutType,
+                state: null,
+              },
+            },
+          }
+        : singleLeafTree();
+    ps.layouts.push({ id: layoutId, name, kind, pluginOrigin: pluginOrigin ?? null, tree });
     return { layoutId };
   }