OG hero images across all five projects

Added hero SVG OG images to all five project landing pages. Each project now has og.svg in its static directory and og:image + twitter:image meta tags pointing to it.

Replaced malachite's three per-page OG SVGs (og-home, og-about, og-import) with a single hero SVG. Upgraded opal, bismuth, and tourmaline from twitter:card summary to summary_large_image.

The hero SVGs are the same ones used in the croft.click portfolio — 1600×900 format with the project logo centred, name and tagline below, and a subtle horizontal rule.