feat(web): whole-page image drop, projected intake limits, and thumbnail tiling

Second alignment step for #2248: document-level drag intake behind the new
DropOverlay atom, lightbox close icon and shared dialog mask, DeepSeek Chat
thumbnail rules (single 240px long edge with ratio clamp, 64px tiles, merged
consecutive assistant images), image limits raised to 20/10MiB/100MiB and
published to clients as the imageLimits projection, whole-batch intake
pre-check with product-copy banners, and attachment-error reasons mapped to
localized copy.

packages/client/ui-attachment/src/ImageLightbox.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef } from 'react'
 import { createPortal } from 'react-dom'
+import { IconCloseOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import css from './ImageLightbox.module.css'
 
 /** Lightbox strings the owner resolves from its own locale namespace. */
@@ -51,10 +52,12 @@ export function ImageLightbox({ src, alt, labels, onClose }: {
       role="dialog"
       aria-modal="true"
       aria-label={labels.dialog}
-      onMouseDown={(event) => { if (event.target === event.currentTarget) onClose() }}
     >
+      <div className={css.mask} aria-hidden="true" onMouseDown={onClose} />
       <img className={css.image} src={src} alt={alt} />
-      <button ref={closeRef} type="button" className={css.close} aria-label={labels.close} onClick={onClose}>×</button>
+      <button ref={closeRef} type="button" className={css.close} aria-label={labels.close} onClick={onClose}>
+        <IconCloseOutline16 size={16} />
+      </button>
     </div>,
     document.body,
   )