feat: add img shortcode template

feat: add img shortcode template

Ported img shortcode from previous theme. Creates responsive, optimized images with LQIP fade-in effect, WebP with JPEG fallback, and responsive srcsets.

Requires imageSizes config in hugo.toml, e.g.:
[params]
imageSizes = [640, 900, 1200, 1600]

Usage: {{< img src="path/to/image.jpg" alt="Description" >}}

Co-Authored-By: Claude Haiku 4.5 <redacted>