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.lock

@@ -1367,6 +1367,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "07bbe89c50d7a535e539b8c17bc0b49bdb77747034daa8087407d655f3f7cc1d"
 dependencies = [
  "futures-core",
+ "futures-sink",
 ]
 
 [[package]]
@@ -3442,6 +3443,7 @@ dependencies = [
  "tauri-plugin-updater",
  "tauri-plugin-window-state",
  "tokio",
+ "url",
 ]
 
 [[package]]
@@ -3737,6 +3739,7 @@ checksum = "eddd3ca559203180a307f12d114c268abf583f59b03cb906fd0b3ff8646c1147"
 dependencies = [
  "base64 0.22.1",
  "bytes",
+ "futures-channel",
  "futures-core",
  "futures-util",
  "http",