Generate devcontainers from project descriptions

[Tian1996]

/claim #14

Summary

• Allow the existing generator input to accept either a GitHub repository URL or a plain-language project description.
• Build a structured Project Description context for non-URL descriptions and cache generated results by a stable description hash.
• Update the prompt source wording, form copy, and client-side validation so project descriptions are accepted without changing the public route.

Tests

• python3 -m unittest tests/test_natural_language_helpers.py
• python3 -m unittest discover -s tests -p 'test_*.py'
• python3 -m py_compile helpers/natural_language_helpers.py main.py helpers/devcontainer_helpers.py tests/test_natural_language_helpers.py
• node --check js/main.js
• git diff --check