New Feature: support auto-install and configuration for FFmpeg and Gifski
- Add settings to customize paths for FFmpeg and Gifski executables
- Auto-download and install dependencies when not found or paths are invalid
- On startup or manual trigger, check for local dependencies; prompt and auto-download if missing
- Show download progress and status to keep users informed
- Validate executables after installation; display error if validation fails
- Update documentation: add “Dependency Installation” section explaining auto-install logic and configuration
