chore: extension icons from the E&P logo, ep_theme is stable

The icon is the E&P signet on a square canvas. ep_theme's
Resources/Public is build output, so its icon is copied there by
the asset build from Resources/Private/Assets/icons.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/ep_theme/ext_emconf.php

@@ -8,7 +8,7 @@ $EM_CONF[$_EXTKEY] = [
     'category' => 'plugins',
     'author' => 'Björn Fromme',
     'author_email' => '[email protected]',
-    'state' => 'beta',
+    'state' => 'stable',
     'dividers2tabs' => true,
     'version' => '3.0.0',
     'constraints' => [