feat: add custom marker icons with configurable size

- Add markerIcon files field to marker.yml for custom JPG/PNG/SVG icons
- Add markerIconSize range field (20-500px, default 40px) with unit display
- Layout icon fields side-by-side (50/50 width) in marker blueprint
- Add markerIconUrl prop in index.php to auto-detect uploaded icon
- Add markerIconSize prop in index.php to read size from page data
- Update MapPreview.vue to display custom images instead of default pins
- Set icon dimensions dynamically based on markerIconSize value
- Icon size updates on save/reload (reactive implementation deferred)
- Remove custom tiles background functionality (not needed)

Note: Custom icons show uploaded image, may have white background on
transparent PNGs depending on image processing. Size is non-reactive
and requires save + reload to update in preview.

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

public/site/blueprints/pages/marker.yml

@@ -55,3 +55,24 @@ tabs:
                 type: map-editor
                 mode: single
                 help: Déplacez le marqueur
+              markerIcon:
+                label: Icône personnalisée
+                type: files
+                multiple: false
+                accept:
+                  - image/jpeg
+                  - image/png
+                  - image/svg+xml
+                width: 1/2
+                help: Image à utiliser comme marqueur (JPG, PNG ou SVG). Laissez vide pour utiliser le marqueur par défaut.
+
+              markerIconSize:
+                label: Taille de l'icône
+                type: range
+                min: 20
+                max: 500
+                step: 5
+                default: 40
+                after: px
+                width: 1/2
+                help: Taille de l'icône en pixels