fix: expand PHP extension checks (#22)

BackupEngine: check ext-zip, ext-pdo, ext-pdo_mysql, ext-mbstring
before running (was only zip + mbstring).

Installer preflight: warn about missing extensions (zip, pdo,
pdo_mysql, mbstring, curl) during install/update. Warns but does
not block installation so the component can still be configured.

MokoRestore already checks ext-zip, ext-pdo_mysql, ext-mbstring,
ext-json in its preflight step.

composer.json already declares all six extensions as requirements
(zip, pdo, pdo_mysql, curl, ftp, mbstring) — composer install
fails if any are missing, which CI enforces.

Closes #22

source/script.php

@@ -58,6 +58,19 @@ class Pkg_MokoSuiteBackupInstallerScript
 			return false;
 		}
 
+		// Check required PHP extensions (warn but don't block install)
+		$requiredExts = ['zip', 'pdo', 'pdo_mysql', 'mbstring', 'curl'];
+		$missingExts  = array_filter($requiredExts, fn($ext) => !extension_loaded($ext));
+
+		if (!empty($missingExts)) {
+			Factory::getApplication()->enqueueMessage(
+				'<strong>MokoSuiteBackup — Missing PHP Extensions</strong>: '
+				. implode(', ', array_map(fn($e) => 'ext-' . $e, $missingExts))
+				. '. Some features (backup, restore, remote upload, notifications) may not work until these are enabled.',
+				'warning'
+			);
+		}
+
 		// Save download key before Joomla re-registers the update site
 		if ($type === 'update') {
 			$this->preflight_saveKey();