The admin panel had a "Create System Message" page (admin.php?p=sysmessage
-> Admin/Templates/sysmessage.tpl) whose form posts to
GameEngine/Admin/Mods/sysmessage.php, but that backend file never existed,
so submitting returned a 404 (Not Found).
Add the handler implementing the prepare -> confirm -> execute flow used by
the template. On execute it displays a global system message to all players
using the existing mechanism (writes Templates/text.tpl from
text_format.tpl and sets users.ok = 1), same as the legacy sysmsg.php.
%TEKST% is escaped for the PHP double-quoted string context to avoid
breakage/injection.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>