Generate covers for pages without a cover

After todo covers landed, the home page still showed five cards with nothing above the title: /features/, /daler/, /reports/, /glossary/ and /reports/audit-meta-refactoring/. PostPreview only drew a generated picture for items with a todo state; an article without cover fell through every branch.

Approach

• src/lib/page-cover.js renders the shared generative background seeded by the page path, with a hue from the same seed. getPathColor() is also what the OG image uses, so a page's listing cover and its social card match
• src/routes/cover/[...path]/+server.js prerenders cover/<path>.svg for every page that has neither cover nor state
• PostPreview falls back to that image when no cover was passed; todo items keep their state-coloured cover and badge

Tasks

• Add the module with tests and the prerendered route
• Add the fallback branch in PostPreview
• Update CLAUDE.md