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/deezer_crypto.rs

@@ -97,6 +97,69 @@ pub fn decrypt_track(data: &[u8], track_id: &str) -> Vec<u8> {
     result
 }
 
+/// Streaming decryption state for processing data chunk-by-chunk
+pub struct StreamingDecryptor {
+    blowfish_key: Vec<u8>,
+    buffer: Vec<u8>,
+}
+
+impl StreamingDecryptor {
+    const CHUNK_SIZE: usize = 2048;
+    const WINDOW_SIZE: usize = Self::CHUNK_SIZE * 3; // 6144
+
+    pub fn new(track_id: &str) -> Self {
+        Self {
+            blowfish_key: generate_blowfish_key(track_id),
+            buffer: Vec::new(),
+        }
+    }
+
+    /// Process incoming data and return decrypted output
+    /// May return less data than input as it buffers to maintain 6144-byte windows
+    pub fn process(&mut self, data: &[u8]) -> Vec<u8> {
+        self.buffer.extend_from_slice(data);
+        let mut output = Vec::new();
+
+        // Process complete windows
+        while self.buffer.len() >= Self::WINDOW_SIZE {
+            let encrypted_chunk = &self.buffer[0..Self::CHUNK_SIZE];
+            let plain_part = &self.buffer[Self::CHUNK_SIZE..Self::WINDOW_SIZE];
+
+            let decrypted = decrypt_chunk(encrypted_chunk, &self.blowfish_key);
+            output.extend_from_slice(&decrypted);
+            output.extend_from_slice(plain_part);
+
+            self.buffer.drain(0..Self::WINDOW_SIZE);
+        }
+
+        output
+    }
+
+    /// Finalize decryption and return any remaining buffered data
+    pub fn finalize(self) -> Vec<u8> {
+        if self.buffer.is_empty() {
+            return Vec::new();
+        }
+
+        let remaining = self.buffer.len();
+
+        if remaining >= Self::CHUNK_SIZE {
+            // Partial window: decrypt first 2048 bytes, keep rest as-is
+            let encrypted_chunk = &self.buffer[0..Self::CHUNK_SIZE];
+            let plain_part = &self.buffer[Self::CHUNK_SIZE..];
+
+            let decrypted = decrypt_chunk(encrypted_chunk, &self.blowfish_key);
+            let mut output = Vec::with_capacity(remaining);
+            output.extend_from_slice(&decrypted);
+            output.extend_from_slice(plain_part);
+            output
+        } else {
+            // Less than 2048 bytes: keep as-is
+            self.buffer
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;