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/plugins/PluginLayoutCellView.tsx

@@ -0,0 +1,73 @@
+/**
+ * `PluginLayoutCellView` — renders a `customPluginLayout` top-level
+ * {@link LayoutNode} (ticket #43, F4, carnet v2 §3): the plugin's registered
+ * React component when its provider is loaded and declares the type,
+ * otherwise {@link PluginLayoutFallback}.
+ *
+ * State is opaque to the domain (`unknown`) and round-trips through
+ * `LayoutGateway` exactly like any other node field — `setState` here is the
+ * only way a plugin component is meant to persist it (never direct gateway
+ * calls from inside a plugin component).
+ */
+
+import type { CustomPluginLayoutCell, PluginAdmin } from "@/domain";
+import { useGateways } from "@/app/di";
+import { usePluginRuntime } from "./PluginRuntimeProvider";
+import { PluginLayoutFallback } from "./PluginLayoutFallback";
+import { resolvePluginLayoutAvailability } from "./layoutAvailability";
+
+interface PluginLayoutCellViewProps {
+  projectId: string;
+  cell: CustomPluginLayoutCell;
+  installedPlugins?: PluginAdmin[];
+  onStateChange: (nextState: unknown) => void;
+  onOpenPlugins: () => void;
+  onChooseAnotherLayout: () => void;
+}
+
+export function PluginLayoutCellView({
+  projectId,
+  cell,
+  installedPlugins,
+  onStateChange,
+  onOpenPlugins,
+  onChooseAnotherLayout,
+}: PluginLayoutCellViewProps) {
+  const { registry } = usePluginRuntime();
+  const gateways = useGateways();
+  const availability = resolvePluginLayoutAvailability(registry, cell, installedPlugins);
+
+  if (availability !== "available") {
+    const providerDisplayName =
+      registry.get(cell.pluginId)?.displayName ??
+      installedPlugins?.find((p) => p.id === cell.pluginId)?.displayName;
+    return (
+      <PluginLayoutFallback
+        cell={cell}
+        availability={availability}
+        providerDisplayName={providerDisplayName}
+        onOpenPlugins={onOpenPlugins}
+        onChooseAnotherLayout={onChooseAnotherLayout}
+      />
+    );
+  }
+
+  const Component = registry.layoutComponent(cell.pluginId, cell.layoutType)!;
+  return (
+    <Component
+      projectId={projectId}
+      nodeId={cell.id}
+      layoutType={cell.layoutType}
+      state={cell.state}
+      setState={onStateChange}
+      availability="available"
+      gateways={{
+        project: gateways.project,
+        git: gateways.git,
+        terminal: gateways.terminal,
+        agents: gateways.agent,
+        system: gateways.system,
+      }}
+    />
+  );
+}