Add PracticeLoop to Music Theory Teaching

[dancourse]

Adds PracticeLoop, a free web tool for musicians to practice difficult passages by slowing down and looping YouTube videos.

Key features:

• Speed control (0.25x-2x) with 4 presets
• A-B loop with keyboard shortcuts
• Progressive speed training (auto-increment after N loop reps)
• Built-in metronome with tap tempo
• Practice timer with session tracking and streaks
• PWA support (add to home screen)
• Instrument-specific landing pages (guitar, bass, piano, drums, violin)

Fits well in Music Theory Teaching section alongside other interactive learning tools.

[dancourse]

Hi! Just a friendly ping on this PR. PracticeLoop is a free web app for musicians to slow down and loop sections of YouTube videos for practice. It's been live for a few months now with growing usage.

Is there anything I can improve to get this merged? Happy to adjust the description or placement if needed.

Thanks for maintaining this great list!