feat(block2): global 'you're offline' banner (network state)

Feedback: offline, the social layer stops working with no clear reason. A slim
amber strip now shows above every screen when there's no network (connectivity_
plus), so it's obvious WHY sharing is paused — the inventory keeps working.
Wrapped once via the MaterialApp builder; connectivity check is guarded so it
no-ops where the plugin is absent (tests/unsupported platforms). i18n en/es/pt.
Analyzer clean.

apps/app_seeds/lib/i18n/pt.i18n.json

@@ -8,7 +8,8 @@
     "delete": "Eliminar",
     "edit": "Editar",
     "type": "Tipo",
-    "comingSoon": "Em breve"
+    "comingSoon": "Em breve",
+    "offline": "Sem ligação — a partilha está em pausa"
   },
   "home": {
     "tagline": "Partilha e cultiva sementes locais",