Use Img component for author avatars

Author avatars are stored in static/images/authors/ and the avatar field is defined in src/lib/authors.js, but PublicationAuthors.svelte doesn't render them yet. Use the common Img component so avatars go through the same image pipeline as everything else (srcset, LQIP).

Current State

• Author interface in src/app.d.ts has an optional avatar field
• src/lib/authors.js has avatar paths set (e.g. /images/authors/ramil.avif)
• PublicationAuthors.svelte only renders author names, no images

Tasks

• Update PublicationAuthors.svelte to render avatar via Img component
• Look up full author data from authors map using the tag
• Handle missing avatars gracefully (initials fallback or hide image)
• Migrate from export let to $props() while at it (Svelte 5)