fix(visual): replace std mutex with async mutex in solar cache

[thep0y]

• Switch cache mutex from std::sync::Mutex to tokio::sync::Mutex for async compatibility
• Update cache initialization to use async Mutex with get_or_init
• Change cache locking to await the async mutex lock instead of blocking
• Avoid potential deadlocks and improve concurrency in solar configuration caching
• Maintain cached solar angles for theme directories asynchronously