Add Spotify-Toolkit to GUI Development section

[DarksAces]

Project

Spotify-Toolkit

Checklist

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• Entry format: - [project-name](url) - Description ending with period.
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Why This Project Is Awesome

Which criterion does it meet? (pick one)

• Industry Standard - The go-to tool for a specific use case
• Rising Star - 5000+ stars in < 2 years, significant adoption
• Hidden Gem - Exceptional quality, solves niche problems elegantly

Explain:
Spotify-Toolkit is a fully functional, open-source desktop suite built with Python 3, CustomTkinter, and Spotipy designed for power users managing large Spotify libraries.

Entry added to README.md:
- [Spotify-Toolkit](https://github.com/DarksAces/Spotify-Toolkit) - Desktop application built with CustomTkinter to manage playlists, clean duplicates, and export library metadata.

Key technical and practical highlights:

• Production-Ready & Native GUI: Features a custom dark-mode interface with a built-in interactive console (rerouting stdout/stdin), live progress tracking, and i18n support (English/Spanish).
• Asynchronous Execution: Multi-threaded execution model that ensures the GUI remains responsive during long-running Spotify API operations.
• Rich Feature Set: Modular tools for bulk duplicate deletion, genre/artist organization, smart shuffling, and playlist metadata export (CSV/JSON).
• Plug & Play CI/CD Architecture: Utilizes GitHub Actions for secret injection during PyInstaller builds, offering end-users a seamless experience without manual API setup while keeping credentials safe.

How It Differs

If similar entries exist, what makes this one unique?
Unlike basic CLI scripts or web wrappers, Spotify-Toolkit provides a complete cross-platform desktop application that combines complex library maintenance (duplicate detection, smart sorting, time calculations) with a modern CustomTkinter user interface, serving as a clean reference implementation for standalone Python desktop tools.