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/en.i18n.json

@@ -5,6 +5,13 @@
     "illustration": "Or pick an illustration",
     "remove": "Remove"
   },
+  "favorites": {
+    "title": "Favorites",
+    "empty": "No favorites yet. Save offers you like from the market.",
+    "save": "Save to favorites",
+    "remove": "Remove from favorites",
+    "unavailable": "No longer available"
+  },
   "seedSaving": {
     "title": "Saving its seed",
     "subtitle": "What it takes to keep the variety true",