Add a warning about register_ui_init (removed from V3 plugin API)

```
➜ python ./scripts/migrate_plugin.py ~/src/picard-plugins/plugins/post_tagging_actions/__init__.py /tmp/post_tagging_actionsv3
Migrating plugin: Post Tagging Actions
  Author: Giorgio Fontanive
  Version: 0.1
  Created: /tmp/post_tagging_actionsv3/MANIFEST.toml
  Created: /tmp/post_tagging_actionsv3/__init__.py
  Regenerated: actions_status.py (from actions_status.ui)
  Regenerated: options_post_tagging_actions.py (from options_post_tagging_actions.ui)

✓ Copied 4 file(s)
✓ Copied 1 directory(ies)

======================================================================
MIGRATION WARNINGS - Manual Review Required:
======================================================================
✓ Converted log.* calls to api.logger.*
✓ Removed 'options' class attribute from PostTaggingActionsOptions (not needed in V3)
✓ Converted config.setting to api.global_config.setting
⚠️  MANUAL MIGRATION REQUIRED: register_ui_init pattern detected
   v2: register_ui_init(...)
   v3: register_ui_init was removed
   See: docs/Plugin2to3MigrationGuide.md - register_ui_init was removed

ℹ️  API access pattern:
   - Processors: Use 'api' parameter (first argument)
   - Classes: Use 'self.api' (passed to __init__)
   - Add api parameter to OptionsPage/BaseAction __init__
✓ Removed Track import - use api.Track instead
✓ Removed Album import - use api.Album instead
✓ Updated BaseAction import to plugin3 API
✓ Updated OptionsPage import to plugin3 API
⚠️  Renamed 2 conflicting file(s):
   options_post_tagging_actions.py → options_post_tagging_actions.py.orig
   actions_status.py → actions_status.py.orig
======================================================================

Migration complete! Plugin saved to: /tmp/post_tagging_actionsv3

Next steps:
  1. Review the generated code in __init__.py
  2. Address all warnings above
  3. Update function signatures as needed
  4. Test the plugin with Picard 3.0
  5. See docs/PLUGINSV3/MIGRATION.md for details
```

Author: zas

scripts/migrate_plugin.py

@@ -746,6 +746,14 @@ def convert_plugin_code(content, metadata):
         all_warnings.append("   See: docs/Plugin2to3MigrationGuide.md - Pattern 1: Album Background Tasks")
         all_warnings.append("")
 
+    # Check for deprecated register_ui_init
+    if 'register_ui_init' in content:
+        all_warnings.append("⚠️  MANUAL MIGRATION REQUIRED: register_ui_init pattern detected")
+        all_warnings.append("   v2: register_ui_init(...)")
+        all_warnings.append("   v3: register_ui_init was removed")
+        all_warnings.append("   See: docs/Plugin2to3MigrationGuide.md - register_ui_init was removed")
+        all_warnings.append("")
+
     # Convert api.* to self.api.* in class methods
     content = convert_api_in_classes(content)