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

admin/config/app.php

@@ -12,6 +12,7 @@ return [
     'content_dir' => __DIR__ . '/../../content/',
     'config_json' => __DIR__ . '/../../config.json',
     'plugins_dir' => __DIR__ . '/../../plugins/',
+    'assets_dir' => __DIR__ . '/../../content/-assets/',
     'admin_config' => __DIR__ . '/admin.json',
     'log_file' => __DIR__ . '/../storage/logs/admin.log',
 ];