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/src/lib.rs

@@ -36,7 +36,7 @@ fn read_audio_metadata(path: String) -> Result<metadata::AudioMetadata, String>
     metadata::read_audio_metadata(&path)
 }
 
-/// Decrypt Deezer track data
+/// Decrypt Deezer track data (legacy - kept for backwards compatibility)
 #[tauri::command]
 async fn decrypt_deezer_track(data: Vec<u8>, track_id: String) -> Result<Vec<u8>, String> {
     // Run decryption on a background thread to avoid blocking the UI
@@ -49,6 +49,89 @@ async fn decrypt_deezer_track(data: Vec<u8>, track_id: String) -> Result<Vec<u8>
     Ok(result)
 }
 
+/// Download and decrypt a Deezer track, streaming directly to disk
+#[tauri::command]
+async fn download_and_decrypt_track(
+    url: String,
+    track_id: String,
+    output_path: String,
+    is_encrypted: bool,
+) -> Result<(), String> {
+    use tokio::io::AsyncWriteExt;
+    use tokio::fs::File;
+    use deezer_crypto::StreamingDecryptor;
+
+    // Build HTTP client
+    let client = reqwest::Client::builder()
+        .timeout(std::time::Duration::from_secs(60))
+        .build()
+        .map_err(|e| format!("Failed to create HTTP client: {}", e))?;
+
+    // Start download
+    let response = client
+        .get(&url)
+        .header("User-Agent", "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.130 Safari/537.36")
+        .send()
+        .await
+        .map_err(|e| format!("Download failed: {}", e))?;
+
+    if !response.status().is_success() {
+        return Err(format!("HTTP error: {}", response.status()));
+    }
+
+    // Open output file
+    let mut file = File::create(&output_path)
+        .await
+        .map_err(|e| format!("Failed to create output file: {}", e))?;
+
+    // Stream download with optional decryption
+    if is_encrypted {
+        let mut decryptor = StreamingDecryptor::new(&track_id);
+        let mut stream = response.bytes_stream();
+
+        use futures_util::StreamExt;
+
+        while let Some(chunk_result) = stream.next().await {
+            let chunk = chunk_result.map_err(|e| format!("Download stream error: {}", e))?;
+
+            // Decrypt chunk and write to file
+            let decrypted = decryptor.process(&chunk);
+            if !decrypted.is_empty() {
+                file.write_all(&decrypted)
+                    .await
+                    .map_err(|e| format!("Failed to write to file: {}", e))?;
+            }
+        }
+
+        // Write any remaining buffered data
+        let final_data = decryptor.finalize();
+        if !final_data.is_empty() {
+            file.write_all(&final_data)
+                .await
+                .map_err(|e| format!("Failed to write final data: {}", e))?;
+        }
+    } else {
+        // No encryption - just stream directly
+        let mut stream = response.bytes_stream();
+
+        use futures_util::StreamExt;
+
+        while let Some(chunk_result) = stream.next().await {
+            let chunk = chunk_result.map_err(|e| format!("Download stream error: {}", e))?;
+            file.write_all(&chunk)
+                .await
+                .map_err(|e| format!("Failed to write to file: {}", e))?;
+        }
+    }
+
+    // Ensure all data is flushed
+    file.flush()
+        .await
+        .map_err(|e| format!("Failed to flush file: {}", e))?;
+
+    Ok(())
+}
+
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
 pub fn run() {
     let library_migrations = vec![Migration {
@@ -171,7 +254,8 @@ pub fn run() {
             greet,
             tag_audio_file,
             read_audio_metadata,
-            decrypt_deezer_track
+            decrypt_deezer_track,
+            download_and_decrypt_track
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");