Restore AppController extension for BC compatibility

[dereuromark]

Summary

Provides a dual solution for controller inheritance:

• Default (BC): QueueAppController extends App\Controller\AppController, inheriting app authentication, components, and configuration
• Standalone opt-in: Set Queue.standalone to true for an isolated admin that skips the app's AppController setup

Usage

// config/app_local.php or config/app.php

// Default: inherits from app's AppController (BC)
// No configuration needed

// Standalone mode: isolated admin, doesn't depend on app
'Queue' => [
    'standalone' => true,
],

The layout configuration via Queue.adminLayout is preserved in both modes.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.30%. Comparing base (241f31d) to head (b57b90b).
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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #464      +/-   ##
============================================
+ Coverage     77.29%   77.30%   +0.01%     
- Complexity      884      885       +1     
============================================
  Files            44       44              
  Lines          3061     3063       +2     
============================================
+ Hits           2366     2368       +2     
  Misses          695      695              

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