Limit image widths and fetch priority

Current State

• IMAGE_WIDTHS in vite.config.js applies to every import. ramil.avif (the 48px avatar) ships as ramil.MXAkiuyc.avif at 44 KB, the largest asset in build/_app/immutable/assets
• Img.svelte filters the srcset client-side with sizes, but the files are still generated and deployed
• PostPreview.svelte renders every card image with loading="eager" (via Articles) and fetchpriority="high"; with 91 cards on /todos/ the hint is meaningless
• role="presentation" is set on card images that also carry alt={post_title} — one of the two is wrong

Tasks

• Give avatars an explicit directive (?w=48;96) or a separate defaultDirectives rule by path
• Trim IMAGE_WIDTHS to the breakpoints the layout actually hits after the grid fix
• Eager + high priority only for the first row of a listing; lazy for the rest
• Use empty alt (the title is next to the image) and drop role