Add inline image support (Kitty, iTerm2, Sixel)

[meszmate]

Closes #32

• Add Kitty graphics protocol image rendering (t=d and t=f with base64 chunking)
• Add iTerm2 inline images via OSC 1337 (with multipart for large files)
• Add Sixel support (pre-encoded .sixel files or img2sixel conversion)
• Unified image_file command that auto-selects the best available protocol
• Runtime protocol detection via terminal queries and env hints
• Placement options: cursor, top_left, top_center, center with row/col offsets
• supportsImages() / drawImageFromFile() on Context for easy usage
• Updated hello_world example with responsive image demo
• Added assets/cat.png demo image