Add region management

This adds region management: the user can manage a list of available regions
and for each channel pick a region from that list to apply to messages.

Region discovery from nearby repeaters will be done in a separate PR.

This is a part of the work needed for #120.

lib/screens/region_management_screen.dart

@@ -0,0 +1,160 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:meshcore_open/connector/meshcore_connector.dart';
+import 'package:meshcore_open/l10n/l10n.dart';
+import 'package:meshcore_open/storage/region_store.dart';
+import 'package:provider/provider.dart';
+
+Future<void> pushRegionManagementScreen(BuildContext context) {
+  return Navigator.push(
+    context,
+    MaterialPageRoute<void>(
+      builder: (context) => const RegionManagementScreen(),
+    ),
+  );
+}
+
+class RegionManagementScreen extends StatefulWidget {
+  const RegionManagementScreen({super.key});
+
+  @override
+  State<RegionManagementScreen> createState() => _RegionManagementScreenState();
+}
+
+class _RegionManagementScreenState extends State<RegionManagementScreen> {
+  final RegionStore _regionStore = RegionStore();
+  List<Region> _regions = [];
+
+  String region = '';
+
+  @override
+  void initState() {
+    super.initState();
+    final connector = context.read<MeshCoreConnector>();
+    _regionStore.setPublicKeyHex = connector.selfPublicKeyHex;
+    _loadRegions();
+  }
+
+  void _loadRegions() {
+    context.read<MeshCoreConnector>().loadChannelSettings();
+
+    final regions = _regionStore.loadRegions();
+    if (mounted) {
+      setState(() {
+        _regions = regions;
+      });
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = context.l10n;
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(l10n.settings_regionManagement_screenTitle),
+        centerTitle: true,
+        actions: [
+          IconButton(
+            tooltip: l10n.settings_regionAddRegion,
+            icon: const Icon(Icons.add),
+            onPressed: () => _showAddRegionDialog(context),
+          ),
+        ],
+      ),
+      body: ListView.builder(
+        padding: const EdgeInsets.only(left: 16, right: 16, top: 8, bottom: 88),
+        itemCount: _regions.length,
+        itemBuilder: (context, index) {
+          final region = _regions[index];
+          return _buildRegionTile(context, region);
+        },
+      ),
+    );
+  }
+
+  void _showAddRegionDialog(BuildContext context) {
+    final l10n = context.l10n;
+    final controller = TextEditingController(text: region);
+    showDialog(
+      context: context,
+      builder: (context) => AlertDialog(
+        title: Text(l10n.settings_regionName),
+        content: TextField(
+          controller: controller,
+          autofocus: true,
+          textInputAction: TextInputAction.send,
+          onSubmitted: (_) => _handleAddRegion(controller.text, context),
+          decoration: InputDecoration(
+            hintText: l10n.settings_regionNameHint,
+            border: const OutlineInputBorder(),
+          ),
+          inputFormatters: <TextInputFormatter>[
+            FilteringTextInputFormatter.allow(RegExp("[a-z0-9-]")),
+          ],
+          maxLength: 30,
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(context),
+            child: Text(l10n.common_cancel),
+          ),
+          TextButton(
+            onPressed: () => _handleAddRegion(controller.text, context),
+            child: Text(l10n.common_add),
+          ),
+        ],
+      ),
+    );
+  }
+
+  void _handleAddRegion(Region region, BuildContext context) {
+    Navigator.pop(context);
+    _regionStore.addRegion(region);
+    _loadRegions();
+  }
+
+  Widget _buildRegionTile(BuildContext context, Region region) {
+    return Card(
+      key: ValueKey(region),
+      child: ListTile(
+        dense: false,
+        title: Text(region),
+        trailing: IconButton(
+          icon: Icon(Icons.delete_outline),
+          onPressed: () => _confirmDelete(context, region),
+        ),
+      ),
+    );
+  }
+
+  void _confirmDelete(BuildContext context, Region region) {
+    showDialog(
+      context: context,
+      builder: (dialogContext) => AlertDialog(
+        title: Text(context.l10n.settings_deleteRegion),
+        content: Text(context.l10n.settings_deleteRegionConfirm(region)),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(dialogContext),
+            child: Text(context.l10n.common_cancel),
+          ),
+          TextButton(
+            onPressed: () async {
+              Navigator.pop(dialogContext);
+              await _regionStore.removeRegion(region);
+              _loadRegions();
+              if (!context.mounted) return;
+              ScaffoldMessenger.of(context).showSnackBar(
+                SnackBar(content: Text(context.l10n.settings_regionDeleted)),
+              );
+            },
+            child: Text(
+              context.l10n.common_delete,
+              style: const TextStyle(color: Colors.red),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}