feat(sessions): show a pending-question indicator on session rows

Adds a per-session pending-question badge to sidebar rows, driven by the
live directory-store question state through a dedicated per-session
subscription channel so unrelated streaming never re-renders rows.

Collapsed parent rows roll up pending questions of hidden descendants
from their owning directory stores without bootstrapping them. Question
state is cloned on session delete/archive so badges clear when sessions
disappear. Adds the questionChangeCallbacks sync performance counter,
i18n keys for all locales, and unit tests for the subscription channel
and scope selection.

Fixes #2634

packages/ui/src/lib/i18n/messages/es.ts

@@ -531,6 +531,8 @@ export const dict: Record<I18nKey, string> = {
   "sessions.sidebar.session.status.pinned": "Sesión anclada",
   "sessions.sidebar.session.status.movingToWorktree": "Moviendo la sesión a un worktree nuevo",
   "sessions.sidebar.session.status.permissionRequired": "Permiso requerido",
+  "sessions.sidebar.session.status.questionPendingSingle": "1 pregunta pendiente",
+  "sessions.sidebar.session.status.questionPendingMany": "{count} preguntas pendientes",
   "sessions.sidebar.session.status.activeFor": "Activa desde hace {duration}",
   "sessions.sidebar.session.status.lastTurnDuration": "El último turno duró {duration}",
   "sessions.sidebar.session.subsessions.collapse": "Colapsar subsesiones",