Markdown images: adaptive 3-column grid, lightbox modal, and broken-image handling ...

- useMarkdown.js: custom marked renderer wraps images in clickable links with
  inline onerror for broken-image detection
- app.scss: .msg-assistant-content becomes flex-wrap container; image paragraphs
  sized to 3-per-row via calc((100% - 16px) / 3); images are square via
  aspect-ratio: 1 / 1 and object-fit: cover
- ImageLightbox.vue: rewritten with proper gnexus-ui-kit .modal/.modal-dialog
  DOM structure; broken images show error placeholder inside modal
- AssistantMessage.vue: attachImageLightbox() wired after v-html render
- Broken images: .is-broken class prevents lightbox click; CSS shows 🖼️ icon

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>