refactor: move download/decryption to backend to fix UI freezing

Now implements streaming download+decryption entirely in Rust:
- Added reqwest/tokio/futures-util dependencies
- Created StreamingDecryptor for chunk-by-chunk decryption
- New download_and_decrypt_track command streams to disk directly
- Frontend simplified to single invoke() call

src-tauri/Cargo.toml

@@ -33,4 +33,7 @@ tauri-plugin-process = "2"
 blowfish = "0.9"
 md5 = "0.7"
 byteorder = "1.5.0"
+reqwest = { version = "0.12.23", features = ["stream", "rustls-tls"] }
+tokio = { version = "1.47.1", features = ["fs", "io-util"] }
+futures-util = "0.3.31"