Fix font variant detection in Google Fonts downloader

[Copilot]

The install_google_font() function hardcoded filenames as {font_name}-Regular.ttf, causing mismatches when Google Fonts serves Bold, Italic, or other variants.

Changes:

• Parse URL to extract variant: Detect Bold, Italic, Light, BoldItalic from the font URL returned by Google Fonts CSS
• Dynamic filename: Save fonts using the detected variant instead of hardcoded 'Regular'
• Case-insensitive matching: Normalize font names for reliable comparison across different naming conventions
• Compound variant precedence: Check 'bolditalic' before 'bold' or 'italic' to avoid incorrect detection
• Variant-aware existence check: Look for any existing variant to skip re-downloads

Example:

# Before: Always saved as "Roboto-Regular.ttf" even if URL pointed to Bold
# After: Parses URL and saves as "Roboto-Bold.ttf" if URL contains Bold variant

# URL: https://fonts.gstatic.com/.../Roboto-Bold.ttf
font_path = install_google_font("Roboto")  
# Saves as: assets/fonts/Roboto-Bold.ttf (matches actual variant)

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