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/plugins/map-editor/index.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * Map Editor Plugin for Kirby CMS
+ *
+ * Interactive map editor with MapLibre GL JS for creating
+ * print-ready maps with markers and rich content.
+ */
+
+Kirby::plugin('geoproject/map-editor', [
+    'fields' => [
+        'map-editor' => [
+            'props' => [
+                'defaultCenter' => function ($defaultCenter = [43.836699, 4.360054]) {
+                    return $defaultCenter;
+                },
+                'defaultZoom' => function ($defaultZoom = 13) {
+                    return $defaultZoom;
+                },
+                'maxMarkers' => function ($maxMarkers = 50) {
+                    return $maxMarkers;
+                }
+            ]
+        ]
+    ]
+]);