Add warning banner to both main and admin pages when running in DEBUG

## Feature Request

**Is your feature request related to a problem? Please describe.**
No. More of a cosmetic solution as a warning to forum owners that they have DEBUG enabled in config.php. Obviously, the banner should only appear to those with admin rights, and who are logged in.

**Describe the solution you'd like**
A detailed description of your proposed solution. Include:
As above. Should be relatively simple in the sense that any code should look for "enabled" in config.php for DEBUG, then show a previously hidden banner warning the user / owner

**Justify why this feature belongs in Flarum's core, rather than in a third-party extension**
DEBUG is part of core, so this should be included in core itself rather than a third party extension

**Describe alternatives you've considered**
No alternatives exist apart from the user getting prompted about DEBUG if an error occurs.

Start by locating config.php and tracing how the main and admin pages render and identify logged-in administrators. Confirm how the core DEBUG setting is read and where a hidden warning banner could be shown on both pages. Done means the banner appears only for logged-in admins when DEBUG is enabled and remains absent otherwise.

Written by the indexing model from the issue text.