feat: use ukiryu as command API for inkscape and ghostscript

[ronaldtse]

This PR adds a SystemCommand module to centralize cross-platform command execution concerns.

Changes

• Create SystemCommand module with three main utilities:

  • format_path: Format paths for command execution (Windows backslash conversion + quoting)
  • headless_environment: Get env vars for headless operation (disable DISPLAY on Unix)
  • find_executable: Find executables in PATH with PATHEXT handling

• Refactor InkscapeWrapper to use:

  • SystemCommand.format_path instead of external_path
  • SystemCommand.headless_environment instead of headless_environment
  • SystemCommand.find_executable instead of manual executable search

• Refactor GhostscriptWrapper to use:

  • SystemCommand.find_executable instead of find_in_path

• Remove duplicated methods from wrappers:

  • external_path (was in InkscapeWrapper)
  • headless_environment (was in InkscapeWrapper)
  • find_in_path (was in GhostscriptWrapper)

Benefits

• Single namespace for cross-platform command execution utilities
• Cleaner wrappers - focused on tool-specific logic, not platform quirks
• DRY - eliminates duplicated path formatting and executable discovery code
• Clear separation - Platform for OS detection, SystemCommand for command execution