fix(plugins): support export default { activate }, cleanup partiel et erreurs visibles

Le loader accepte désormais aussi la forme export default { activate }
en plus de l'export nommé, les subscriptions partiellement établies
sont nettoyées en cas d'échec d'activation, et une erreur de
runtime-catalog est remontée visuellement dans PluginsPanel avec un
badge Invalide/Isolé sur les plugins concernés (#116/#120).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/plugins/PluginRuntimeProvider.tsx

@@ -38,6 +38,13 @@ const EMPTY_PLUGIN_RUNTIME: PluginRuntimeContextValue = {
 
 const PluginRuntimeContext = createContext<PluginRuntimeContextValue>(EMPTY_PLUGIN_RUNTIME);
 
+function describeError(e: unknown): string {
+  if (e && typeof e === "object" && "message" in e) {
+    return String((e as { message: unknown }).message);
+  }
+  return String(e);
+}
+
 interface PluginRuntimeProviderProps {
   children: ReactNode;
   /** Test/Storybook escape hatch — skips the gateway fetch and uses this value as-is. */
@@ -72,10 +79,19 @@ export function PluginRuntimeProvider({ children, value: injected }: PluginRunti
         if (cancelled) return;
         setValue({ registry: result.registry, failures: result.failures, loading: false });
       })
-      .catch(() => {
+      .catch((e: unknown) => {
         // No plugin gateway / catalog fetch failed: run with zero plugins
         // rather than blocking the app (full-trust plugins are additive).
-        if (!cancelled) setValue((prev) => ({ ...prev, loading: false }));
+        if (!cancelled) {
+          setValue((prev) => ({
+            ...prev,
+            failures: [
+              ...prev.failures,
+              { pluginId: "<runtime-catalog>", reason: describeError(e) },
+            ],
+            loading: false,
+          }));
+        }
       });
     return () => {
       cancelled = true;