feat: add install-theme command for Beamer presentations

[prudhomm]

Summary

Add install-theme command to download and install Beamer themes for presentations.

Changes

• New: src/article_cli/themes.py - ThemeInstaller class with download and extraction logic
• New: tests/test_themes.py - 31 tests for theme functionality
• Modified: src/article_cli/config.py - Added get_themes_config() method
• Modified: src/article_cli/cli.py - Added install-theme command
• Modified: README.md - Theme installation documentation

Usage

# List available themes
article-cli install-theme --list

# Install numpex theme
article-cli install-theme numpex

# Install to custom directory
article-cli install-theme numpex --dir themes/

# Install from custom URL
article-cli install-theme my-theme --url https://example.com/theme.zip

Features

• Download themes from configured URLs (GitHub releases, archives)
• Extract specific .sty files and directories (e.g., images/)
• Built-in support for numpex theme from presentation.template.d
• Configurable via [themes] section in config file
• Automatic detection of font/engine requirements

Testing

• 31 new tests for theme installation (all passing)
• mypy: ✅ No issues
• black: ✅ Formatted

Closes #10