feat: Use XDG config directory on Linux

[ArnabTechiee]

Description

This change updates the application to use the standard XDG config directory (~/.config/spotdl) on Linux, while maintaining backward compatibility for existing users.

Related Issue

Closes #2387

Motivation and Context

This helps de-clutter the user's home folder on Linux and follows standard conventions, as requested in issue #2387.

How Has This Been Tested?

I have manually tested the following scenarios:

1. New user: Verified the new ~/.config/spotdl directory is created.
2. Existing user (legacy): Verified the old ~/.spotdl directory is used if present.
3. Existing user (modern): Verified the new ~/.config/spotdl is prioritized if both directories exist.

Screenshots (if appropriate)

Types of Changes

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my changes
• All new and existing tests passed

[Copilot]

Pull Request Overview

This PR implements XDG Base Directory Specification compliance for Linux systems by changing the default config directory from ~/.spotdl to ~/.config/spotdl, while maintaining backward compatibility for existing users.

• Updates get_spotdl_path() to use XDG config directory (~/.config/spotdl) on Linux
• Implements backward compatibility logic to check for existing ~/.spotdl directory
• Maintains current behavior for non-Linux systems

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[Silverarmor]

Remove import platformdirs

[Silverarmor]

Looks great, thanks for your submission!

In the future, please update documentation and ensure tests pass. Instructions in CONTRIBUTING.md