fix(ios): prevent UI freeze from sync artwork fetch and duration access

[erik-lissen]

Fixes #147

Summary

Fixes two iOS blocking issues that cause the app to freeze on slow/bad networks:

1. Artwork fetch blocked threads - Data(contentsOf:) and semaphore-based URLSession calls blocked GCD threads while fetching artwork for the lock screen. On failure, onError() tore down the entire player. Now uses fully async URLSession with a 10s timeout and emits a soft PLAYBACK_ERROR on failure.

2. item.asset.duration.seconds blocked on HLS - Accessing AVAsset.duration synchronously forces metadata loading, which requires fetching and parsing the m3u8 playlist. Removed this synchronous access; duration is already set via progress events once the player item is loaded.

3. Artwork is now optional - Empty or missing artwork no longer causes validation failures or player errors. The AudioProTrack.artwork field is now optional in the TypeScript type and all JS/native validation.

Reproduction

1. Use Network Link Conditioner set to "Very Bad Network"
2. Play any HLS track with a remote artwork URL
3. Before fix: entire app freezes for 5-30 seconds
4. After fix: audio plays immediately, artwork loads asynchronously when available

Changes

• src/types.ts - Make artwork optional in AudioProTrack
• src/utils.ts - Allow empty artwork in validation
• ios/AudioPro.swift - Three changes:
  • Extract artwork from required guard statement
  • Replace sync artwork fetch with async URLSession + 10s timeout
  • Remove sync item.asset.duration.seconds access

Test plan

• Play track with artwork - artwork appears on lock screen
• Play track without artwork - no crash, audio plays
• Play on very bad network - no freeze
• Rapid track switching on bad network - no freeze
• Lock screen controls work correctly
• Duration shows correctly on lock screen after load