feat: add map framing save button and prevent initial load save

- Add "Définir le cadrage" button in multi mode to save current map
  center and zoom as default framing
- Add getCurrentZoom() method in MapPreview to retrieve current zoom level
- Fix: prevent automatic save on initial load with isInitialLoad flag
  to avoid marking page as modified when loading existing data
- Style: dark button theme for save framing button

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/site/plugins/map-editor/src/components/map/MapPreview.vue

@@ -309,6 +309,13 @@ export default {
       };
     }
 
+    function getCurrentZoom() {
+      if (map.value && map.value.loaded()) {
+        return map.value.getZoom();
+      }
+      return props.zoom;
+    }
+
     function centerOnPosition(lat, lon) {
       if (map.value && map.value.loaded()) {
         map.value.flyTo({
@@ -323,6 +330,7 @@ export default {
       mapContainer,
       loading,
       getCurrentCenter,
+      getCurrentZoom,
       centerOnPosition
     };
   }