feat: add offline test mode for locally saved questions

[ericklucioh]

I added an offline test mode to leetgo in my fork, ericklucioh/leetgo (https://github.com/ericklucioh/leetgo), so questions can be tested
later without depending on the LeetCode API at test time.

What changed:

• leetgo pick now stores local offline metadata for the generated question.
• The saved data includes:
  • slug
  • frontend id
  • language
  • output directory
  • generated filenames
  • question content
  • translated content
  • question metadata
• A new offline test flow was added:
  • leetgo test -O
• Offline testing works only with local artifacts already generated by pick.
• The offline flow still supports -t to run a subset of cases.

Examples:

leetgo pick 1
leetgo test -O last
leetgo test -O last -t 1-3

Current behavior and limitations:

• Offline state is stored per ProjectRoot(), so it is tied to the repository where pick was run.
• Because of that, the offline cache is not automatically shared across different clones or forks.
• Offline mode does not support system design questions yet.
• If required metadata is missing, leetgo pick must be run again.
• -O cannot be combined with --local, --both, --submit, or --force.