feat(logging): add verbose mode and debug output

[CalvinAllen]

Summary

• Add --verbose global flag to enable debug output for all commands
• Add DTVEM_VERBOSE environment variable for debugging shim behavior
• Add ui.Debug() function with timestamps for debug messages
• Add debug output to download, extract, install, list, and shim operations
• Improve error messages with URL and file path context for better troubleshooting
• Add comprehensive tests for verbose mode functionality

Usage

# Enable verbose mode via flag
dtvem --verbose install python 3.11.0

# Enable verbose mode for shim debugging
DTVEM_VERBOSE=1 python --version

Test plan

• Unit tests for verbose mode toggle
• Unit tests for environment variable checking
• Unit tests for debug output
• Manual test of --verbose flag
• All existing tests pass

Closes #92