fix: add Fastlane UI import to changelog_helper

- Changed from Fastlane.const_defined?(:UI) to defined?(UI)
- Fixes NameError: uninitialized constant ChangelogHelper::UI

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: graycreate

fastlane/changelog_helper.rb

@@ -5,7 +5,7 @@
 
 module ChangelogHelper
   # Import Fastlane's UI for logging
-  UI = FastlaneCore::UI unless Fastlane.const_defined?(:UI)
+  UI = FastlaneCore::UI unless defined?(UI)
   # Extract changelog for a specific version from CHANGELOG.md
   # @param version [String] The version to extract (e.g., "1.1.1")
   # @return [String] The changelog content for the specified version