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superseded multimodal note and package docs.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/tsdown.config.ts

@@ -1,5 +1,9 @@
 import { clientOnly } from '../tsdown.client.ts'
 
+// TODO(client-atoms): verbatim copy of ui-primitives/tsdown.config.ts (only
+// the package differs). On a third atoms package, extract a shared css-stub
+// client-library preset in packages/client/tsdown.client.ts instead of a
+// fourth copy.
 /**
  * ui-attachment is browser-only, but its lib bundle IS imported under plain
  * Node because the web shell is a lib (dsh-client-web's lib chain reaches