feat: add CLI main module with Click integration

Implemented the CLI entry point using Click framework. The CLI:
- Accepts Space URL as an argument
- Supports --cookies and --output options
- Validates URLs using the validator module
- Downloads Spaces using the downloader module
- Provides clear success and error messages
- Uses proper exit codes (0=success, 1=validation error, 2=download error)

Test uses Click's CliRunner to test CLI without running actual downloads.
Mocks validate_space_url and download_space functions for isolated testing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_main.py

@@ -0,0 +1,21 @@
+import pytest
+from click.testing import CliRunner
+from unittest.mock import patch, Mock
+from src.main import cli
+
+
+def test_cli_valid_url(tmp_path):
+    """Test CLI with valid URL."""
+    runner = CliRunner()
+
+    with patch('src.main.validate_space_url') as mock_validate, \
+         patch('src.main.download_space') as mock_download:
+
+        mock_validate.return_value = (True, "1ZkKzZWnVRRKv", None)
+        mock_download.return_value = str(tmp_path / "space_1ZkKzZWnVRRKv.mp3")
+
+        result = runner.invoke(cli, ['https://x.com/i/spaces/1ZkKzZWnVRRKv'])
+
+        assert result.exit_code == 0
+        assert "Successfully downloaded" in result.output
+        assert "space_1ZkKzZWnVRRKv.mp3" in result.output