feat: Add auto rename functionality to both post and creator downloaders
- Add auto rename checkbox to both downloader tabs with default enabled state
- Implement sequential file numbering (1_filename, 2_filename, etc.) when enabled
- Maintain original filename structure while adding numerical prefix for organization
- Update UI translations to support auto rename feature across all language files
- Preserve file extensions and handle edge cases for filename sanitization
- Add per-post file counter tracking for creator downloader to ensure proper sequencing