feat: album cover art in Discord Rich Presence via iTunes Search API

Fetches album artwork from the iTunes Search API and passes the URL
directly to Discord's large_image field. Subsonic cover URLs require
auth and can't be used by Discord directly.

- 3-strategy search: exact quoted → relaxed → track title fallback
- 1-hour in-memory cache per artist|album key
- iTunes search runs in tokio::task::spawn_blocking (reqwest::blocking
  must not run on the Tokio async executor)
- artwork_cache wrapped in Arc<Mutex<...>> for cross-thread sharing
- Fallback to the pre-uploaded "psysonic" asset when no match found
- Cargo.toml: blocking feature alongside rustls-tls

Co-Authored-By: kilyabin <kilyabin@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -32,7 +32,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 rodio = { version = "0.19", default-features = false, features = ["symphonia-all"] }
 symphonia = { version = "0.5", default-features = false, features = ["flac", "mp3", "pcm", "aac", "isomp4", "vorbis", "ogg", "wav", "adpcm"] }
-reqwest = { version = "0.12", default-features = false, features = ["stream", "json", "multipart", "rustls-tls"] }
+reqwest = { version = "0.12", default-features = false, features = ["stream", "json", "multipart", "rustls-tls", "blocking"] }
 futures-util = "0.3"
 md5 = "0.7"
 tokio = { version = "1", features = ["rt", "time"] }
@@ -43,3 +43,4 @@ tauri-plugin-updater = "2"
 tauri-plugin-process = "2"
 souvlaki = { version = "0.8", default-features = false, features = ["use_zbus"] }
 discord-rich-presence = "0.2"
+url = "2"