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/ui/app_drawer.dart

@@ -103,6 +103,12 @@ class AppDrawer extends StatelessWidget {
                   _DrawerItem(
                     icon: const Icon(Icons.favorite),
                     label: t.menu.wishlist,
+                    onTap: marketEnabled
+                        ? () {
+                            Navigator.of(context).pop();
+                            context.push('/favorites');
+                          }
+                        : null,
                   ),
                   // The ego-centric web of trust ("your people") — live once the
                   // social layer is on.