feat(web): toast anchoring and model-selection rejection banner

The toast sits 120px from the viewport top and centers over its anchor —
the composer card, so it centers on the chat column rather than the window;
a rejected model selection (e.g. picking a text-only model while the
session holds images) announces through the same banner while the in-menu
strip with Retry stays the catalog-load surface. The attachment rail
consumes every wheel tick with a vertical component: a diagonal pan keeps
its horizontal intent and nothing scrolls the conversation behind the
composer.

packages/client/ui-primitives/src/Toast.module.css

@@ -6,7 +6,7 @@
 
 .toast {
   position: fixed;
-  top: 80px;
+  top: 120px;
   left: 50%;
   /* Above the 1000 the image lightbox backdrop uses: a failure reported while
      a preview is open must stay readable. */