fix: move postinstall string to package sys.ini

COM_MOKOBACKUP_POSTINSTALL_UPDATE_SITE was showing as a raw key
after install because the component language file isn't loaded
during package installation. Moved to PKG_MOKOBACKUP_POSTINSTALL_UPDATE_SITE
in pkg_mokobackup.sys.ini where the installer can resolve it.

Authored-by: Moko Consulting
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/language/en-GB/pkg_mokobackup.sys.ini

@@ -7,3 +7,4 @@
 PKG_MOKOBACKUP="Package - MokoJoomBackup"
 PKG_MOKOBACKUP_DESCRIPTION="Full-site backup and restore for Joomla — database, files, and configuration. Includes admin component, system plugin, and REST API."
 PKG_MOKOBACKUP_PHP_VERSION_ERROR="MokoJoomBackup requires PHP %s or later."
+PKG_MOKOBACKUP_POSTINSTALL_UPDATE_SITE="MokoJoomBackup installed successfully. Configure your <a href=\"%s\">Update Site</a> to receive automatic updates."

src/language/en-US/pkg_mokobackup.sys.ini

@@ -7,3 +7,4 @@
 PKG_MOKOBACKUP="Package - MokoJoomBackup"
 PKG_MOKOBACKUP_DESCRIPTION="Full-site backup and restore for Joomla — database, files, and configuration. Includes admin component, system plugin, and REST API."
 PKG_MOKOBACKUP_PHP_VERSION_ERROR="MokoJoomBackup requires PHP %s or later."
+PKG_MOKOBACKUP_POSTINSTALL_UPDATE_SITE="MokoJoomBackup installed successfully. Configure your <a href=\"%s\">Update Site</a> to receive automatic updates."

src/script.php

@@ -194,7 +194,7 @@ class Pkg_MokoBackupInstallerScript
 				);
 
 				Factory::getApplication()->enqueueMessage(
-					Text::sprintf('COM_MOKOBACKUP_POSTINSTALL_UPDATE_SITE', $editUrl),
+					Text::sprintf('PKG_MOKOBACKUP_POSTINSTALL_UPDATE_SITE', $editUrl),
 					'info'
 				);
 			}