Preload metadata for embedded CUE tracks

[markojurkovic]

Summary

This PR extends playlist metadata preloading to tracks generated from embedded CUE sheets.

Embedded CUE tracks retain the underlying audio file extension, such as album.flac#01. Previously, only external CUE URLs such as album.cue#01 were recognized, which could cause embedded-CUE rows to briefly display incomplete or raw metadata.

Implementation

• Recognizes external .cue track URLs as before.
• For other fragmented file URLs, inspects the fragment-free source file for embedded CUE metadata.
• Checks both metadata and audio properties for a non-empty CUE sheet.
• Avoids treating every fragmented URL as a CUE track, preserving correct behavior for unrelated multi-subsong formats.

Test plan

• Confirm embedded-CUE tracks display their metadata immediately when inserted.
• Confirm external CUE sheets continue to preload correctly.
• Confirm non-CUE subsong URLs are not misclassified.
• Build Cog in the macOS Debug configuration.

The full macOS Debug build passes.