feat: add map-editor plugin with interactive OSM map and markers

Implement Phase 1 of custom Kirby plugin for editing interactive maps:
- OpenStreetMap base layer with MapLibre GL JS
- Click to add markers, drag to reposition
- Marker list sidebar with selection and deletion
- Auto-save with debounce (YAML format)
- Add marker button creates marker at current map center
- Max 50 markers per map (configurable)
- Clean UI with marker counter

Blueprint updated to use new map-editor field type instead of placeholder.

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

public/site/blueprints/pages/map.yml

@@ -13,12 +13,12 @@ columns:
           text: 
             label: Présentation de la carte
             type: writer
-          map: 
+          mapdata:
             label: Carte
-            type: info
-            text: |
-              Ici le plugin pour la carte et les marqueurs
-              Avoir la possibilité de changer le fond de carte en image
+            type: map-editor
+            defaultCenter: [43.836699, 4.360054]
+            defaultZoom: 13
+            maxMarkers: 50
   sidebar:
     width: 1/3
     sections: