Fix: show all plugins in content-edit, not just those with plugin.json

HTMLBlock has no plugin.json, only HTMLBlock.php.
Accept plugin if it has plugin.json OR <PluginName>.php.

public/admin.php

@@ -508,14 +508,18 @@ function handleContentEdit($auth, $config, $twig, $user, $csrf, $siteConfig): vo
         }
     }
 
-    // Get available plugins
+    // Get available plugins (must have plugin.json or <PluginName>.php)
     $pluginsDir = $config['plugins_dir'];
     $availablePlugins = [];
     if (is_dir($pluginsDir)) {
         foreach (glob($pluginsDir . '*', GLOB_ONLYDIR) as $pluginDir) {
             $pluginName = basename($pluginDir);
-            if ($pluginName !== '.' && $pluginName !== '..' && file_exists($pluginDir . '/plugin.json')) {
-                $availablePlugins[] = $pluginName;
+            if ($pluginName !== '.' && $pluginName !== '..') {
+                $hasJson = file_exists($pluginDir . '/plugin.json');
+                $hasPhp = file_exists($pluginDir . '/' . $pluginName . '.php');
+                if ($hasJson || $hasPhp) {
+                    $availablePlugins[] = $pluginName;
+                }
             }
         }
     }