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/blueprints/pages/map.yml

@@ -7,10 +7,10 @@ columns:
       fields:
         type: fields
         fields:
-          tags: 
+          tags:
             label: Mots-clés
             type: tags
-          text: 
+          text:
             label: Présentation de la carte
             type: writer
           mapdata:
@@ -25,6 +25,3 @@ columns:
       files:
         label: Fichiers
         type: files
-
-
-

public/site/blueprints/pages/marker.yml

@@ -0,0 +1,63 @@
+title: Marqueur
+icon: location
+
+tabs:
+  content:
+    label: Contenu
+    columns:
+      main:
+        width: 1/1
+        sections:
+          fields:
+            type: fields
+            fields:
+              title:
+                label: Titre du marqueur
+                type: text
+                required: true
+              content:
+                label: Contenu
+                type: blocks
+                fieldsets:
+                  - heading
+                  - text
+                  - image
+                  - list
+                  - quote
+
+  position:
+    label: Position
+    columns:
+      left:
+        width: 1/3
+        sections:
+          coordinates:
+            type: fields
+            fields:
+              latitude:
+                label: Latitude
+                type: number
+                step: 0.000001
+                min: -90
+                max: 90
+                required: true
+              longitude:
+                label: Longitude
+                type: number
+                step: 0.000001
+                min: -180
+                max: 180
+                required: true
+      right:
+        width: 2/3
+        sections:
+          map:
+            type: fields
+            fields:
+              mapPreview:
+                label: Position sur la carte
+                type: map-editor
+                mode: single
+                latitude: "{{ page.latitude }}"
+                longitude: "{{ page.longitude }}"
+                help: Déplacez le marqueur ou recherchez une adresse