feat(market): save others' offers as favorites (Wallapop-style)

Adds a Favorites feature: a heart on the offer detail saves another
person's listing to an encrypted, per-identity SavedOffersStore
(keystore JSON snapshot, no plaintext at rest). A new /favorites screen
(wired from the drawer) lists saved offers offline-first and, when a
relay is reachable, flags ones that are gone as "no longer available".
i18n en/es/pt/ast; store + screen + detail-heart tests.

apps/app_seeds/lib/i18n/es.i18n.json

@@ -5,6 +5,13 @@
     "illustration": "O elige un dibujo",
     "remove": "Quitar"
   },
+  "favorites": {
+    "title": "Favoritos",
+    "empty": "Aún no tienes favoritos. Guarda ofertas que te gusten del mercado.",
+    "save": "Guardar en favoritos",
+    "remove": "Quitar de favoritos",
+    "unavailable": "Ya no está disponible"
+  },
   "seedSaving": {
     "title": "Conservar su semilla",
     "subtitle": "Lo que hace falta para mantener la variedad fiel",