Fix #29: validate --container against known containers

Before this fix, passing an invalid container name (e.g., 'nonexistent')
was passed directly to FFmpeg, which would fail with a cryptic error.
Now the container name is validated against CONTAINER_NAMES before any
processing begins, and a clear error message listing valid containers
is shown.

audio_splitter/main.py

@@ -32,6 +32,7 @@ from .defaults import (
     DEFAULT_SKIP_EXISTING,
     DEFAULT_DELETE_ORIGINAL,
 )
+from .constants import CONTAINER_NAMES
 from .core import split_audio
 from .tracklist import read_tracklist, parse_format
 from .ffmpeg import has_cover_or_video, validate_cover_images
@@ -196,6 +197,12 @@ def main():
         print(f"Error: Tracklist file not found: {args.tracklist_file}")
         sys.exit(1)
 
+    # Validate container if provided
+    if args.container and args.container not in CONTAINER_NAMES:
+        print(f"Error: Invalid container '{args.container}'. "
+              f"Valid containers: {', '.join(sorted(CONTAINER_NAMES))}.")
+        sys.exit(1)
+
     # Check FFmpeg
     try:
         subprocess.run(['ffmpeg', '-version'], capture_output=True, check=True)