feat: transform map-editor markers into Kirby subpages

Major refactoring of the map-editor plugin to store markers as Kirby
subpages instead of YAML data, enabling extensible block content.

Backend Changes:
- Add API routes for marker CRUD operations (GET, POST, PATCH, DELETE)
- Create marker.yml blueprint with content & position tabs
- Add markers section to map.yml blueprint
- Update useMapData to only handle center/zoom/background
- Create useMarkersApi composable for API communication

Frontend Changes:
- Refactor MapEditor.vue to support multi/single modes
- Multi mode: loads markers via API, redirects to Panel for editing
- Single mode: displays single marker for position tab in marker page
- Remove MarkerEditor.vue modal (replaced by Panel editing)
- Normalize position format handling (lon vs lng)

API Features:
- Session-based auth for Panel requests (no CSRF needed)
- Proper error handling and validation
- Markers created as listed pages (not drafts)
- Uses Kirby's data() method for JSON parsing

Documentation:
- Add IMPLEMENTATION_SUMMARY.md with technical details
- Add TESTING_CHECKLIST.md with 38 test cases

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

public/site/plugins/map-editor/index.php

@@ -19,8 +19,20 @@ Kirby::plugin('geoproject/map-editor', [
                 },
                 'maxMarkers' => function ($maxMarkers = 50) {
                     return $maxMarkers;
+                },
+                'mode' => function ($mode = 'multi') {
+                    return $mode;
+                },
+                'latitude' => function ($latitude = null) {
+                    return $latitude;
+                },
+                'longitude' => function ($longitude = null) {
+                    return $longitude;
                 }
             ]
         ]
+    ],
+    'api' => [
+        'routes' => require __DIR__ . '/api/routes.php'
     ]
 ]);