feat(help-center): add help panel to sidebar icon bar

New help-center module adds a question-mark icon to the sidebar with
an aside panel containing three sections: Documentation (permission-
gated links to docs viewer and API docs), Keyboard Shortcuts (inline
reference from HotkeyService with Mac/Win variants), and About
(application info). Visible to all logged-in users — individual
resource links respect existing permissions.

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

config/modules.php

@@ -12,5 +12,5 @@
  * Each entry must match a directory name under modules/ containing a module.php manifest.
  */
 return [
-    'enabled_modules' => ['audit', 'addressbook', 'bookmarks', 'notifications', 'api-docs'],
+    'enabled_modules' => ['audit', 'addressbook', 'bookmarks', 'notifications', 'api-docs', 'help-center'],
 ];