feat(player): share one lyrics panel across player layouts

[mihaimetal]

Summary

• Extract PlayerMediaPanel so the side player and the bottom-bar lyrics popover render the same lyrics surface.
• The popover keeps its source-picker header; the side player stays headerless until a later chapters change needs tabs.

Why

Both layouts were independently wiring LyricsBody + a one-off header. A shared panel keeps them in sync and is the hook the chapters PR hangs tabs on.

Scope

Frontend-only, rebased cleanly on v0.4.4 / current origin/main. No playback, auth, or stream-resolve commits.

Test plan

• pnpm test
• tsc --noEmit
• Side player: lyrics still flow under the cover, source picker still in the bottom row
• Bottom-bar lyrics popover: title + source picker + lyrics body
• No track: lyrics button stays disabled

Notes

Independent of the macOS / WEB_REMIX / speed PRs. The follow-up chapters PR (feature/chapters) stacks on this commit.

[mihaimetal]

Closing in favor of #72 — this was only the PlayerMediaPanel extract that chapters hangs tabs on. Combined into one PR so there isn't a one-commit stack to review separately.