Add Code settings group: custom admin CSS/JS + header/footer scripts

New Settings > Code group (Custom CSS & JS + Header & footer scripts) driven
by admin/src/Code/CodeInjection.php:
- Admin CSS  → <style> in the admin <head> (</style> stripped to prevent breakout)
- Admin JS   → <script> in the admin footer (</script> neutralized)
- Header scripts → printed verbatim in the front-end <head>
- Footer scripts → printed verbatim before the front-end </body>

Header/footer scripts are raw admin-authored markup by design (analytics,
meta, etc.), settable only by manage_options users on the settings page.
SettingsField gains a `mono` flag for monospace textareas (no CodeMirror
dependency). All four injection points verified on PHP 8.5.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

admin/src/App/AuroraAdmin.php

@@ -47,6 +47,7 @@ use AuroraAdmin\Media\SvgUploads;
 use AuroraAdmin\Media\MediaFolders;
 use AuroraAdmin\ImageFormats\ImageFormats;
 use AuroraAdmin\Security\Security;
+use AuroraAdmin\Code\CodeInjection;
 use AuroraAdmin\Rest\MediaFolderData;
 use AuroraAdmin\Rest\BugReportData;
 use AuroraAdmin\Rest\CompanionPluginData;
@@ -99,6 +100,7 @@ class AuroraAdmin
     new MediaFolders();
     new ImageFormats();
     new Security();
+    new CodeInjection();
     new MediaFolderData();
     new BugReportData();
     new CompanionPluginData();