feat(music): add song download functionality and rename client class

- Implement download_song method in MusicClient with progress tracking
- Rename songboxClient to SongboxClient for consistency
- Update documentation and examples to reflect class name change
- Add download command to CLI interface

README.md

@@ -31,10 +31,10 @@ pip install -e .
 ### Python Library Usage
 
 ```python
-from songbox import songboxClient
+from songbox import SongboxClient
 
 # Create client
-client = songboxClient()
+client = SongboxClient()
 
 # Authenticate
 verify_id = client.auth.login("960", "7777777")
@@ -125,7 +125,7 @@ songbox user me --format table
 ### Client Configuration
 
 ```python
-client = songboxClient(
+client = SongboxClient(
     timeout=30.0,  # Request timeout in seconds
     headers={"Custom-Header": "value"}  # Additional headers
 )
@@ -195,10 +195,10 @@ songbox music trending [--limit <number>]
 ### Complete Authentication Flow
 
 ```python
-from songbox import songboxClient
+from songbox import SongboxClient
 from songbox.exceptions import AuthenticationError
 
-client = songboxClient()
+client = SongboxClient()
 
 try:
     # Step 1: Initiate login