feat: persist MQTT broker settings in store

lib/data/sources/local/credential_store.dart

@@ -9,6 +9,8 @@ class CredentialStore {
   static const _passwordKey = 'localiserd_password';
   static const _hostKey = 'localiserd_host';
   static const _portKey = 'localiserd_port';
+  static const _mqttHostKey = 'mqtt_host';
+  static const _mqttPortKey = 'mqtt_port';
 
   final _storage = const FlutterSecureStorage();
 
@@ -46,6 +48,24 @@ class CredentialStore {
     return ServerConfig(host: host, port: port);
   }
 
+  Future<void> saveMqttBroker(String host, int port) => Future.wait([
+        _storage.write(key: _mqttHostKey, value: host),
+        _storage.write(key: _mqttPortKey, value: port.toString()),
+      ]).then((_) {});
+
+  Future<({String host, int port})?> loadMqttBroker() async {
+    final results = await Future.wait([
+      _storage.read(key: _mqttHostKey),
+      _storage.read(key: _mqttPortKey),
+    ]);
+    final host = results[0];
+    final portStr = results[1];
+    if (host == null || portStr == null) return null;
+    final port = int.tryParse(portStr);
+    if (port == null) return null;
+    return (host: host, port: port);
+  }
+
   Future<void> clear() => Future.wait([
         _storage.delete(key: _usernameKey),
         _storage.delete(key: _passwordKey),

lib/providers.dart

@@ -44,6 +44,12 @@ final credentialStoreProvider = Provider<CredentialStore>((ref) {
   return CredentialStore();
 });
 
+/// MQTT broker settings stored locally on the device.
+/// Null if not yet configured. Set via [CredentialStore.saveMqttBroker].
+final mqttBrokerProvider = FutureProvider<({String host, int port})?>(
+  (ref) => ref.watch(credentialStoreProvider).loadMqttBroker(),
+);
+
 // ---------------------------------------------------------------------------
 // Feature clients — throw if required state is missing
 // ---------------------------------------------------------------------------