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

TODO.md

@@ -0,0 +1,6 @@
+# TODO
+
+## ✅ Voltooid
+- Elk thema zit in zijn eigen map in de thema's map (`themes/naam/theme.json`)
+- In admin → content: alle bestanden kunnen hernoemd worden via de edit-pagina
+- In admin → content: bestanden en mappen kunnen verplaatst worden