Plugin system: content vs system types, sidebar-aware UI, arrow visibility

- plugin.json type field: 'content' (sidebar) or 'system' (API only)
- Content-edit: label 'Plugins', hide section if layout has no sidebar
- Content-edit: disable checkboxes when no sidebar layout selected
- Content-edit: hide up arrow on first item, down arrow on last item
- PluginManager: isPluginViewable() checks type=system -> not viewable
- Admin plugins page: show Content/Systeem type badge
- HTMLBlock: add plugin.json with type=content
- Navigation: add type=content to config

public/admin.php

@@ -508,9 +508,10 @@ function handleContentEdit($auth, $config, $twig, $user, $csrf, $siteConfig): vo
         }
     }
 
-    // Get available plugins (must have plugin.json or <PluginName>.php)
+    // Get available content plugins (type=content in plugin.json, or no type = content)
     $pluginsDir = $config['plugins_dir'];
     $availablePlugins = [];
+    $sidebarLayouts = [];
     if (is_dir($pluginsDir)) {
         foreach (glob($pluginsDir . '*', GLOB_ONLYDIR) as $pluginDir) {
             $pluginName = basename($pluginDir);
@@ -518,12 +519,27 @@ function handleContentEdit($auth, $config, $twig, $user, $csrf, $siteConfig): vo
                 $hasJson = file_exists($pluginDir . '/plugin.json');
                 $hasPhp = file_exists($pluginDir . '/' . $pluginName . '.php');
                 if ($hasJson || $hasPhp) {
-                    $availablePlugins[] = $pluginName;
+                    // Check plugin type — only content plugins show in sidebar
+                    $pluginType = 'content';
+                    if ($hasJson) {
+                        $pj = json_decode(file_get_contents($pluginDir . '/plugin.json'), true);
+                        $pluginType = $pj['type'] ?? 'content';
+                    }
+                    if ($pluginType === 'content') {
+                        $availablePlugins[] = $pluginName;
+                    }
                 }
             }
         }
     }
 
+    // Determine which layouts have a sidebar (based on theme twig files)
+    foreach ($themeLayouts as $key => $twigFile) {
+        if (strpos($key, 'sidebar') !== false) {
+            $sidebarLayouts[] = $key;
+        }
+    }
+
     $currentLang = extractLanguagePrefix($file);
     $route = 'content-edit';
     $fileDir = trim(dirname($file), '.\\/');
@@ -541,6 +557,7 @@ function handleContentEdit($auth, $config, $twig, $user, $csrf, $siteConfig): vo
         'currentLayout' => $currentLayout ?: $themeDefaultLayout,
         'themeLayouts' => $themeLayouts,
         'themeDefaultLayout' => $themeDefaultLayout,
+        'sidebarLayouts' => $sidebarLayouts,
         'activeThemeName' => $activeThemeName,
         'availablePlugins' => $availablePlugins,
         'selectedPlugins' => $selectedPlugins,