Fix multiple bugs and improve UX in file converter

- Fix conversion strategies: all 5 strategies now work correctly
  (smart_auto, all_available, common_only, most_complete, separate)
- Add validation feedback with warning dialogs for missing files,
  empty selections, and insufficient files for merge
- Fix merge remove button to allow removing selected files (not just last)
- Add thread cleanup to prevent resource leaks
- Integrate encoding detection instead of hardcoded UTF-8
- Add overwrite confirmation before replacing existing files
- Remove unused strategy dropdown that did nothing
- Share selected files when switching between Convert/Split/Merge tabs

Author: alexanderv-sifi