Generate todo cover images

Todo pages have no photo, so the listings showed a gradient rectangle with a state label. It carried the state colour and the urgency tint, but every todo looked the same. The goal: a real picture per todo, stable across builds, without hand-made assets.

Approach

• src/lib/todo-cover.js is a pure module: state palette, urgency (0..10 from the page date relative to a reference date), a seeded PRNG from the slug hash, and renderTodoCoverSvg() that draws a gradient, blurred hue-shifted blobs, a diagonal band, rings and dots onto a 1020x360 canvas
• src/routes/(demo)/todos/[slug]/cover.svg/+server.js prerenders one SVG per todo page via entries(), so the build contains todos/<slug>/cover.svg next to each page
• TodoCover.svelte now places that image and overlays the state badge; PostPreview passes the item link so the component can find the cover
• Covered by todo-cover.test.js: urgency mapping, colour helpers, determinism per slug

Tasks

• Move state colours, labels and urgency into a pure module
• Add the generative renderer and its tests
• Add the prerendered cover.svg route
• Render the image from TodoCover, keep the badge
• Update CLAUDE.md and the glossary