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/strings_en.g.dart

@@ -113,6 +113,9 @@ class Translations$common$en {
 
 	/// en: 'Coming soon'
 	String get comingSoon => 'Coming soon';
+
+	/// en: 'You're offline — sharing is paused'
+	String get offline => 'You\'re offline — sharing is paused';
 }
 
 // Path: home
@@ -1755,6 +1758,7 @@ extension on Translations {
 			'common.edit' => 'Edit',
 			'common.type' => 'Type',
 			'common.comingSoon' => 'Coming soon',
+			'common.offline' => 'You\'re offline — sharing is paused',
 			'home.tagline' => 'Share and grow local seeds',
 			'home.openMarket' => 'Market',
 			'home.openMarketSubtitle' => 'Discover and share seeds nearby',