Media browser: recursive scan entire content/ tree, /-media/ URL prefix, editor change detection fix

- handleMediaList() now scans content/ recursively for all media files
- URLs use /-media/ prefix mapping directly to content/ (no special cases)
- index.php: added /-media/ route, kept /-assets/ for backward compat
- editor-toolbar.js: fixed editor.on('change') placement (was inside switchMode)
- content-edit.php and content-new.php: back-btn unsaved-changes detection
- Removed unused __editorCleanup global

themes/default/theme.json

@@ -0,0 +1,13 @@
+{
+    "name": "Standaard",
+    "header_color": "#0a369d",
+    "header_font_color": "#ffffff",
+    "header_height": "56",
+    "navigation_color": "#2754b4",
+    "navigation_font_color": "#ffffff",
+    "nav_height": "42",
+    "sidebar_background": "#f8f9fa",
+    "sidebar_border": "#dee2e6",
+    "background_image": "",
+    "background_image_opacity": "100"
+}

themes/test/theme.json

@@ -0,0 +1,13 @@
+{
+    "name": "test",
+    "header_color": "#613583",
+    "header_font_color": "#ffffff",
+    "navigation_color": "#813d9c",
+    "navigation_font_color": "#ffffff",
+    "sidebar_background": "#f8f9fa",
+    "sidebar_border": "#dee2e6",
+    "header_height": "120",
+    "nav_height": "50",
+    "background_image": "test_bg.jpg",
+    "background_image_opacity": 10
+}