Move the duplicated TYPE_ICONS record from citation.tsx and
citation-list.tsx into a new type-icons.ts module so the mapping lives
in one place and both components import from the same source.
Resolves#1190
- Replace raw <img> with Next.js Image in markdown-viewer.tsx
- Use next/image with fill + sizes in assistant-ui image.tsx
- Optimize favicons with explicit dimensions in citation components
- Set unoptimized=true for data/blob URLs and external favicons
