feat: load Google Maps key dynamically from backend

- MapsService: obtiene la key via API e inyecta el script Maps JS en runtime
- LocationPickerDialog: muestra error descriptivo si la key no esta configurada
- index.html: eliminado script estatico de Maps

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/services/maps_service.dart

@@ -0,0 +1,60 @@
+import 'dart:async';
+// ignore: avoid_web_libraries_in_flutter
+import 'dart:js' as js;
+import 'package:flutter/foundation.dart';
+import 'package:prosapp_web_app/services/api_service.dart';
+
+class MapsService {
+  static bool _loaded = false;
+  static Completer<bool>? _completer;
+
+  static Future<bool> load() async {
+    if (_loaded) return true;
+    if (_completer != null) return _completer!.future;
+
+    _completer = Completer<bool>();
+
+    try {
+      final data = await ApiService.instance.get('/settings/maps-key');
+      final apiKey = data['api_key'] as String? ?? '';
+
+      if (apiKey.isEmpty) {
+        debugPrint('MapsService: API key no configurada');
+        _completer!.complete(false);
+        return false;
+      }
+
+      if (kIsWeb) {
+        _injectScript(apiKey);
+      }
+
+      _loaded = true;
+      _completer!.complete(true);
+      return true;
+    } catch (e) {
+      debugPrint('MapsService: error cargando key — $e');
+      _completer!.complete(false);
+      return false;
+    }
+  }
+
+  static void _injectScript(String apiKey) {
+    // Verifica si ya está cargado
+    final existing = js.context.callMethod('eval', [
+      'typeof google !== "undefined" && typeof google.maps !== "undefined"'
+    ]);
+    if (existing == true) {
+      _loaded = true;
+      return;
+    }
+
+    js.context.callMethod('eval', ['''
+      (function() {
+        var s = document.createElement('script');
+        s.src = 'https://maps.googleapis.com/maps/api/js?key=$apiKey';
+        s.async = true;
+        document.head.appendChild(s);
+      })();
+    ''']);
+  }
+}