[no bump] Shift plugin icon index down by one

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugin.json

@@ -4,7 +4,7 @@
   "description": "Multi-file exteraGram plugin built by tools/bundle.py",
   "author": "@lyuksovannyy",
   "version": "1.0.0",
-  "icon": "exteraGramPluginIcons/1",
+  "icon": "exteraGramPluginIcons/0",
   "app_version": ">=12.5.1",
   "sdk_version": ">=1.4.4.3",
   "requirements": []