Cross-references and unified support links

Added a "More tools" section to each of the five projects (malachite, opal, jasper, bismuth, tourmaline) linking to the other four. The four monorepo projects use a new siblings prop on the landing-ui LandingPage component. Tourmaline has its own inline implementation matching the same card style.

Replaced all ko-fi.com/ewancroft and github.com/sponsors/ewanc26 links with ewancroft.uk/support. Consolidated the landing-ui hero from two separate buttons (Sponsor + Ko-fi) into a single "Support" button. Removed target="_blank" from own-domain links since they're internal navigation.

Also switched tourmaline's "View post" link from bsky.app to aturi.to — universal links that let the viewer pick their preferred ATProto client.