Update inject_ftl.sh with improved FTL/DTD injection logic

Слиты Проскурнёв Илья Сергеевич запросил слияние update-inject-script в main
  • Support multi-line FTL blocks with proper indentation
  • Replace existing keys instead of only adding missing ones
  • Add support for DTD files
  • Support file paths like [chrome/messenger/file.ftl]
  • Add documentation in sync.md

Co-Authored-By: Z-AI noreply@z-ai