fix(help-center): replace inline content with standard navigation panel

Rewrite help panel to use the same grouped navigation pattern as the
admin aside panel (details/summary with nav links). Remove inline
keyboard shortcuts table and about section. Panel now shows
permission-gated navigation groups: Documentation (docs viewer, API
docs) and System (system info). Remove custom CSS — panel inherits
existing app-sidebar-admin-nav styling.

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

modules/help-center/module.php

@@ -3,8 +3,8 @@
 /**
  * Help Center module manifest.
  *
- * Adds a help icon to the sidebar with documentation links,
- * keyboard shortcuts reference, and system information.
+ * Adds a help icon to the sidebar with navigation links
+ * to documentation, API docs, and system information.
  */
 return [
     'id' => 'help-center',
@@ -31,13 +31,6 @@ return [
                 'order' => 900,
             ],
         ],
-        'layout.head_style' => [
-            [
-                'key' => 'help-center-style',
-                'path' => 'modules/help-center/css/components/app-help-panel.css',
-                'order' => 200,
-            ],
-        ],
     ],
 
     'authorization_policies' => [],