feat: save and offer Wi-Fi credentials when adding new sensors

lib/data/sources/local/credential_store.dart

@@ -1,8 +1,11 @@
+import 'dart:convert';
+
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 
 import '../../../domain/models/server_config.dart';
 
 typedef Credentials = ({String username, String password});
+typedef WifiCredential = ({String ssid, String password});
 
 class CredentialStore {
   static const _usernameKey = 'localiserd_username';
@@ -11,6 +14,7 @@ class CredentialStore {
   static const _portKey = 'localiserd_port';
   static const _mqttHostKey = 'mqtt_host';
   static const _mqttPortKey = 'mqtt_port';
+  static const _wifiCredentialsKey = 'wifi_credentials';
 
   final _storage = const FlutterSecureStorage();
 
@@ -66,6 +70,36 @@ class CredentialStore {
     return (host: host, port: port);
   }
 
+  Future<List<WifiCredential>> loadWifiCredentials() async {
+    final raw = await _storage.read(key: _wifiCredentialsKey);
+    if (raw == null) return [];
+    try {
+      final list = jsonDecode(raw) as List<dynamic>;
+      return list
+          .whereType<Map<String, dynamic>>()
+          .map((m) => (
+                ssid: m['ssid'] as String? ?? '',
+                password: m['password'] as String? ?? '',
+              ))
+          .where((c) => c.ssid.isNotEmpty)
+          .toList();
+    } catch (_) {
+      return [];
+    }
+  }
+
+  Future<void> saveWifiCredential(WifiCredential credential) async {
+    final existing = await loadWifiCredentials();
+    final updated = [
+      credential,
+      ...existing.where((c) => c.ssid != credential.ssid),
+    ];
+    final json = jsonEncode(
+      updated.map((c) => {'ssid': c.ssid, 'password': c.password}).toList(),
+    );
+    await _storage.write(key: _wifiCredentialsKey, value: json);
+  }
+
   Future<void> clear() => Future.wait([
         _storage.delete(key: _usernameKey),
         _storage.delete(key: _passwordKey),