Fix localized asset resolution for region codes

[bitomule]

Summary

• Resolve explicit localized asset maps using language-only fallbacks such as es-ES -> es
• Allow BCP-47 style keys like en-US in localized asset maps
• Support nested localized asset layouts like screenshots/{lang}/iphone/01.png

Fixes #33

Tests

• .venv/bin/pytest tests/test_generator.py tests/test_config.py tests/test_localized_assets.py tests/test_html_renderer.py -q
• .venv/bin/pytest tests/ -q