feat(player): add Share to the track menu

[mihaimetal]

Summary

• Add a Share item to the track menu (right-click on a row and the player overflow). It copies https://music.youtube.com/watch?v=<id> to the clipboard.
• Extract copyToClipboard with a hidden-textarea + execCommand("copy") fallback for WebViews that block navigator.clipboard — the same path the artist-page Share button already uses.

Why

Artist pages already have Share. Songs and videos did not, so the only way to grab a link was to leave the app.

Scope

Frontend-only, rebased cleanly on v0.4.4 / current origin/main. Independent of the lyrics (#71), chapters (#72), macOS, WEB_REMIX, and speed PRs.

Test plan

• tsc --noEmit
• Right-click a track → Share → paste is a music.youtube.com/watch?v=… URL
• Player ⋯ menu → Share — same toast and same URL
• Toast is "Link copied to clipboard" on success

Notes

Does not change the artist-page Share button; that already copies the channel URL.