feat(playback): add selectable playback speed

[mihaimetal]

Summary

• Add a discrete playback-speed control on the player chrome (0.1×–2×).
• Persist the chosen rate and apply it via HTMLAudioElement.playbackRate.
• Keep the floating player in sync; scale Discord Rich Presence progress timestamps by the selected rate.

Why

Useful for practice / review / long podcasts without changing the rest of the playback stack. Uses the browser's built-in rate control (pitch is preserved by default).

Scope

Frontend-only, rebased cleanly on current origin/main. Independent of media-key PRs (#43) and auth work.

Test plan

• pnpm test (includes playback.test.ts rate validation)
• tsc --noEmit
• Play a track, open the speed menu, switch 0.5× / 1.25× / 1.5× / 2× — audio rate changes
• Restart app — selected rate is restored
• Floating player reflects rate changes when used
• Discord RP (if enabled) progress bar roughly matches wall-clock at non-1× rates
• Seek / skip / next still work at non-1× rates

Notes

Compatible with NUber-dev/YTubic main as of the PR base; no media-key or auth commits included.