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/layoutAvailability.ts

@@ -0,0 +1,36 @@
+/**
+ * Plugin layout availability resolution (ticket #43, F4, carnet v2 §3.4).
+ *
+ * A `customPluginLayout` top-level {@link LayoutNode} is opaque domain state —
+ * the provider plugin may be missing, disabled, or incompatible with the
+ * running IdeA version at any given session. Availability is never stored in
+ * the layout itself; it's derived here, purely, from the loaded runtime
+ * registry, so it's trivially testable without React.
+ */
+
+import type { CustomPluginLayoutCell, PluginAdmin, PluginLayoutAvailability } from "@/domain";
+import type { PluginRuntimeRegistry } from "@/plugins/runtime";
+
+export function resolvePluginLayoutAvailability(
+  registry: PluginRuntimeRegistry,
+  cell: CustomPluginLayoutCell,
+  /**
+   * Optional admin list (from `PluginGateway.listPlugins()`) to tell "known but
+   * disabled" apart from "not installed at all" — the runtime registry alone
+   * only ever holds *loaded* (enabled) plugins (carnet §1.3), so it can't make
+   * that distinction by itself. Omit it to fall back to "plugin-missing" for
+   * both cases (still correct, just less precise fallback copy).
+   */
+  installedPlugins?: PluginAdmin[],
+): PluginLayoutAvailability {
+  const plugin = registry.get(cell.pluginId);
+  if (!plugin) {
+    const known = installedPlugins?.find((p) => p.id === cell.pluginId);
+    return known && !known.enabled ? "plugin-disabled" : "plugin-missing";
+  }
+  const declaresType = plugin.contributes.layouts.some((l) => l.type === cell.layoutType);
+  if (!declaresType) return "incompatible";
+  const component = plugin.layouts.get(cell.layoutType);
+  if (!component) return "incompatible";
+  return "available";
+}