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/assets/icons/default-marker.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="40" height="40" viewBox="0 0 40 40">
+  <circle cx="20" cy="16" r="14" fill="#e74c3c" stroke="white" stroke-width="3"/>
+  <circle cx="20" cy="16" r="6" fill="white"/>
+  <path d="M 20 30 L 15 20 L 25 20 Z" fill="#e74c3c" stroke="white" stroke-width="2"/>
+</svg>