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/tests/toast.spec.tsx

@@ -1,7 +1,7 @@
 // @vitest-environment jsdom
 
 import { afterEach, describe, expect, it, vi } from 'vitest'
-import { cleanup, render } from '@testing-library/react'
+import { cleanup, fireEvent, render } from '@testing-library/react'
 import { Toast } from '../src/Toast.tsx'
 
 afterEach(cleanup)
@@ -24,6 +24,23 @@ describe('Toast', () => {
     }
   })
 
+  it('centers over its anchor and re-measures on window resize', () => {
+    vi.useFakeTimers()
+    try {
+      const anchor = document.createElement('div')
+      document.body.appendChild(anchor)
+      anchor.getBoundingClientRect = () => ({ left: 100, width: 400 }) as DOMRect
+      const view = render(<Toast text="anchored" anchor={anchor} onDone={vi.fn()} />)
+      expect(view.getByRole('alert').style.left).toBe('300px')
+      anchor.getBoundingClientRect = () => ({ left: 200, width: 400 }) as DOMRect
+      fireEvent(window, new Event('resize'))
+      expect(view.getByRole('alert').style.left).toBe('400px')
+      anchor.remove()
+    } finally {
+      vi.useRealTimers()
+    }
+  })
+
   it('renders without an icon and cancels its timer on unmount', () => {
     vi.useFakeTimers()
     try {