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/plugins/map-editor/index.php

@@ -28,6 +28,23 @@ Kirby::plugin('geoproject/map-editor', [
                 },
                 'longitude' => function ($longitude = null) {
                     return $longitude;
+                },
+                'markerIconUrl' => function ($markerIconUrl = null) {
+                    // Auto-detect marker icon from page files
+                    if ($markerIconUrl === null && $this->model()) {
+                        $iconFile = $this->model()->markerIcon()->toFile();
+                        if ($iconFile) {
+                            return $iconFile->url();
+                        }
+                    }
+                    return $markerIconUrl;
+                },
+                'markerIconSize' => function ($markerIconSize = 40) {
+                    // Auto-detect marker icon size from page
+                    if ($this->model() && $this->model()->markerIconSize()->isNotEmpty()) {
+                        return (int) $this->model()->markerIconSize()->value();
+                    }
+                    return $markerIconSize;
                 }
             ]
         ]