Add settings, markup and docs for track artwork lookup

[tonyzeoli]

Summary

• Some stream types (notably Live365) don't include album art in their metadata feed, only title/artist.
• Adds a "Fetch Album Artwork?" checkbox (Bar tab → Metadata section, default on) plus optional Spotify Client ID/Secret text fields, used as a fallback artwork source. All PRO-gated, matching the existing player-bar-metadata settings pattern.
• Adds a rp-now-playing-artwork <img> element to the player bar's now-playing markup (player/radio-player.php), alongside the existing title/artist/album elements, hidden by default until populated.
• Documents the feature: fills in the previously-empty "Track Metadata" section in docs/Player.md, documents the new settings in docs/Options.md, and extends the existing "Does this plugin use any 3rd party services?" FAQ entry in readme.txt to disclose iTunes/Spotify usage — explicit that Spotify credentials are the site owner's own responsibility to obtain.

This mirrors the equivalent work just done on Radio Station / Radio Station PRO, so both product lines stay in sync. Pairs with a companion PR on stream-player-pro that does the actual iTunes/Spotify lookup and populates this element — this PR only adds the settings surface, markup hook and docs so the free plugin exposes the extension points PRO needs. No behavior change without PRO active.

Note: branched off `master`, not `develop` — `develop` in this repo is 90 commits behind and hasn't been touched since 2023, while active development clearly continues on `master`.

Test plan

• Confirm the new settings render correctly under Settings → Bar tab.
• Confirm the player bar renders the new (hidden) artwork image without visual regression when PRO is inactive.

🤖 Generated with Claude Code