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superseded multimodal note and package docs.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-primitives/src/Toast.module.css

@@ -11,6 +11,10 @@
   /* Above the 1000 the image lightbox backdrop uses: a failure reported while
      a preview is open must stay readable. */
   z-index: 1100;
+  /* Purely an announcement: it must never intercept clicks — in particular
+     after the CSS fade finished while a throttled background-tab timer has
+     not yet unmounted the still-hit-testable fixed element. */
+  pointer-events: none;
   display: flex;
   align-items: center;
   gap: 10px;
@@ -56,3 +60,11 @@
     opacity: 0;
   }
 }
+
+/* Reduced motion drops the slide-in; the delayed fade (an opacity change,
+   not movement) still ends the banner before the timed unmount. */
+@media (prefers-reduced-motion: reduce) {
+  .toast {
+    animation: dsh-toast-fade 1000ms ease 3000ms forwards;
+  }
+}