fix: runAgent fire-and-forget + polling sur la page /agents

Le bouton Play des cartes agents appelait runAgent (Server Action) qui
attendait executeAgent (~2-5 min) → spinner bloqué sans résultat.

- runAgent retourne immédiatement { success, agentId }
- agent-card.tsx lance un polling toutes les 3s sur GET
  /api/agents/run-for-note?agentId= jusqu'au statut terminal
- Toast persistant Sonner pendant la génération, mis à jour au résultat
- Cleanup automatique du polling au démontage

Co-authored-by: Cursor <cursoragent@cursor.com>

memento-note/locales/en.json

@@ -1579,6 +1579,8 @@
       "updated": "Agent updated",
       "deleted": "\"{name}\" deleted",
       "deleteError": "Error deleting",
+      "running": "Generation in progress…",
+      "runningDesc": "Generation may take a few minutes. You can navigate freely.",
       "runSuccess": "\"{name}\" executed successfully",
       "runError": "Error: {error}",
       "runFailed": "Execution failed",