fix(web): review-round attachment refinements

Body-portal the lightbox and toast so transformed ancestors cannot trap
their fixed positioning (a lightbox opened from a chat message covered only
the chat column); make the toast pointer-transparent; observe the rail
element's own size instead of window resizes; consume vertical wheel ticks
exclusively via a non-passive listener with LINE/PAGE delta normalization;
keep the start position when the rail mounts over an existing draft; honor
prefers-reduced-motion for the toast, remove-control, and paging; retry
loads through the guarded load effect; note the deliberate promptError
re-announce; pin the intake toast in the assembled snapshot; sync the
superseded multimodal note and package docs.

packages/client/ui-attachment/src/AttachmentRail.module.css

@@ -81,6 +81,12 @@
   }
 }
 
+@media (prefers-reduced-motion: reduce) {
+  .remove {
+    transition: none;
+  }
+}
+
 .arrow {
   position: absolute;
   top: 50%;