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/ui/offline_banner.dart

@@ -0,0 +1,87 @@
+import 'dart:async';
+
+import 'package:connectivity_plus/connectivity_plus.dart';
+import 'package:flutter/material.dart';
+
+import '../i18n/strings.g.dart';
+
+/// Wraps the app and shows a slim strip at the very top when there's no network,
+/// so it's clear WHY the social layer (market, chat, profile) is paused — the
+/// inventory keeps working offline. Wrap the whole app once (MaterialApp builder).
+class OfflineBanner extends StatefulWidget {
+  const OfflineBanner({required this.child, super.key});
+
+  final Widget child;
+
+  @override
+  State<OfflineBanner> createState() => _OfflineBannerState();
+}
+
+class _OfflineBannerState extends State<OfflineBanner> {
+  bool _offline = false;
+  StreamSubscription<List<ConnectivityResult>>? _sub;
+
+  @override
+  void initState() {
+    super.initState();
+    _init();
+  }
+
+  Future<void> _init() async {
+    try {
+      final current = await Connectivity().checkConnectivity();
+      if (mounted) setState(() => _offline = _isOffline(current));
+      _sub = Connectivity().onConnectivityChanged.listen((results) {
+        if (mounted) setState(() => _offline = _isOffline(results));
+      });
+    } catch (_) {
+      // Platform without connectivity support → assume online, hide the banner.
+    }
+  }
+
+  bool _isOffline(List<ConnectivityResult> results) =>
+      results.isEmpty ||
+      results.every((r) => r == ConnectivityResult.none);
+
+  @override
+  void dispose() {
+    _sub?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: [
+        if (_offline)
+          Material(
+            color: const Color(0xFFE8A200), // amber — not an error, just paused
+            child: SafeArea(
+              bottom: false,
+              child: Padding(
+                padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 6),
+                child: Row(
+                  mainAxisAlignment: MainAxisAlignment.center,
+                  children: [
+                    const Icon(Icons.cloud_off, size: 16, color: Colors.white),
+                    const SizedBox(width: 8),
+                    Flexible(
+                      child: Text(
+                        context.t.common.offline,
+                        style: const TextStyle(
+                          color: Colors.white,
+                          fontSize: 12.5,
+                          fontWeight: FontWeight.w500,
+                        ),
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            ),
+          ),
+        Expanded(child: widget.child),
+      ],
+    );
+  }
+}