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

cms/templates/assets/header.mustache

@@ -1,4 +1,4 @@
-<header class="navbar navbar-expand-lg navbar-dark" style="background-color: var(--header-bg);">
+<header id="site-header" class="navbar navbar-expand-lg navbar-dark" style="background-color: transparent;">
     <div class="container-fluid">
         <a class="navbar-brand" href="?page={{default_page}}&lang={{current_lang}}">
             <img src="/assets/icon.svg" alt="CodePress Logo" width="32" height="32" class="me-2">