feat(frontend): système de plugins — runtime, menus, layouts custom (#43)

Lots F1-F4 : runtime de chargement/registre plugin, extension des menus
existants, panneau de gestion des plugins, types de layout custom
(sélecteur, fallback, cellule dédiée) branchés sur le port plugin.
Suite npm typecheck/test verte (947/947).

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

frontend/src/features/layout/LayoutTabs.tsx

@@ -15,6 +15,7 @@ import { useEffect, useRef, useState } from "react";
 
 import type { LayoutInfo } from "@/domain";
 import { cn } from "@/shared";
+import { PluginLayoutSelectorSection, usePluginRuntime } from "@/features/plugins";
 import { useLayouts } from "./useLayouts";
 
 interface LayoutTabsProps {
@@ -43,6 +44,10 @@ export function LayoutTabs({ projectId, onActiveLayoutChange }: LayoutTabsProps)
   const renameInputRef = useRef<HTMLInputElement | null>(null);
   // Show/hide the create-kind dropdown.
   const [showCreateMenu, setShowCreateMenu] = useState(false);
+  const { registry: pluginRegistry } = usePluginRuntime();
+  // Plugin layouts need a `create_layout` backend extension that doesn't exist
+  // yet (#43, F4 open point) — surfaced rather than silently swallowed.
+  const [pluginLayoutNotice, setPluginLayoutNotice] = useState<string | null>(null);
 
   async function handleSelect(id: string) {
     // The effect above propagates the new active layout (id + kind) to the parent.
@@ -196,13 +201,22 @@ export function LayoutTabs({ projectId, onActiveLayoutChange }: LayoutTabsProps)
             >
               ⎇ Git graph
             </button>
+            <PluginLayoutSelectorSection
+              registry={pluginRegistry}
+              onSelect={(choice) => {
+                setShowCreateMenu(false);
+                setPluginLayoutNotice(
+                  `« ${choice.layout.label} » (${choice.pluginDisplayName}) : la création de layouts plugins nécessite une extension backend pas encore livrée.`,
+                );
+              }}
+            />
           </div>
         )}
       </div>
 
-      {vm.error && (
+      {(vm.error || pluginLayoutNotice) && (
         <span className="ml-2 text-xs text-danger" role="alert">
-          {vm.error}
+          {vm.error ?? pluginLayoutNotice}
         </span>
       )}
     </div>