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>

web/index.html

@@ -16,8 +16,6 @@
   <script type="text/javascript">
     window.flutterWebRenderer = 'html';
   </script>
-  <!-- Google Maps JS API — reemplazar GOOGLE_MAPS_API_KEY con la key real -->
-  <script src="https://maps.googleapis.com/maps/api/js?key=GOOGLE_MAPS_API_KEY"></script>
 
   <style>
     * { margin: 0; padding: 0; box-sizing: border-box; }