Add playback speed control

[henryshuen]

Summary

• Add session-scoped playback speed control from 0.50× to 3.00× with 0.05× adjustments and common presets.
• Apply the selected speed across Legacy, Gapless, and Crossfade playback while preserving it through seek, pause/resume, and track changes.
• Add an anchored, themed playback speed popup to the full playback control bar.

Testing

• npx jest -w 1 --runTestsByPath src/app/ui/components/playback-speed-control/playback-speed-control.component.spec.ts src/app/services/playback/playback.service.spec.ts --noStackTrace
• npm run build:prod
• git diff --check

[digimezzo]

Hi @henryshuen, thank you so much for this pull request. I'll review it in the days to come and will give feedback. Stay tuned! 😉

[digimezzo]

HI @henryshuen, thank you for this amazing feature. I've reviewed you implementation and it looks very clean. I'm merging.