Add standalone /publiser admin application

Implement a minimal, dependency-free publishing tool to manage news
and articles. The app includes:

- A Markdown-based block editor with rich-text capabilities.
- Image upload and resizing using GD.
- Support for drafts, scheduled publishing, and a trash system.
- Integrated Basic Auth protection via PHP and .htpasswd.
- A global plugin for automatic execution of scheduled posts.

The tool reuses core site logic in custom/plugins/publiser-lib.php to
ensure consistency between the editor and the live site.

.gitignore

@@ -10,3 +10,5 @@ content/.user.ini
 content/php.ini
 custom/assets/petition-map-data.json
 custom/data/petition-map-cache.json
+custom/data/publiser/
+content/publiser/.htpasswd