Show raw filename tooltip on Now Playing title

[paulpv]

Summary

Add a hover tooltip to the Now Playing header so hovering the current track shows its raw filename/path, mirroring the playlist row behavior.

Proposed work

• Surface the underlying filename (or path + filename, consistent with playlist tooltips) via the Now Playing element's title attribute.
• Ensure the tooltip updates whenever the active track changes.
• Keep the on-screen label using the friendly short track name so the UI stays compact.
• Verify accessibility: confirm screen readers still announce the friendly name while exposing the tooltip for mouse users.
• Document manual test steps in the README covering long filenames and quick track changes.