Redesign: share images and favicon #34

Part of the redesign build (parent issue linked in the sidebar). Targets the redesign branch. Needs the foundations issue.

The designs and their exact values are in docs/redesign/paper/ on the redesign branch (exported from the Paper file, page "A · Documents").

• Share images, 1200×630: one per article, generated at build time (its title, date and reading time), and one for Home and About. Replaces the single share image used today. The Home and About image uses the updated text given in that folder's README: label "Projects · Writing", and the home intro under the name.
• Favicon: the paper tile (panel #f3f3f0, an ink "m", an orange square dot), as an SVG icon with the glyphs converted to paths, a favicon.ico (16px and 32px) and a 180px apple-touch-icon. Replaces Astro's default logo.