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-model/tests/model-select.spec.tsx

@@ -135,6 +135,38 @@ describe('ModelSelect reasoning effort', () => {
     expect(screen.getByRole('menuitemradio', { name: 'DeepSeek-V4-Flash' })).toBeTruthy()
   })
 
+  it('announces a rejected selection as a transient toast and keeps the in-menu strip for loads', async () => {
+    const groups = [{
+      id: 'deepseek-official',
+      name: 'DeepSeek',
+      models: [
+        { id: 'deepseek-v4-flash', name: 'DeepSeek-V4-Flash', reasoning },
+        { id: 'deepseek-v4-pro', name: 'DeepSeek-V4-Pro' },
+      ],
+    }]
+    const directory = createSnapshotStore<ModelDirectoryState>(state({ groups }))
+    const select = vi.fn(async () => {
+      directory.set(state({ groups, status: 'error', error: 'model-unavailable: session already contains images' }))
+      return false
+    })
+    render(<ModelSelect
+      locked={false}
+      available
+      directory={directory}
+      load={vi.fn()}
+      select={select}
+      t={t}
+    />)
+
+    fireEvent.click(screen.getByRole('button', { name: /选择模型|当前/ }))
+    fireEvent.click(screen.getByRole('menuitem', { name: /模型/ }))
+    fireEvent.click(screen.getByRole('menuitemradio', { name: /DeepSeek-V4-Pro/ }))
+    const toast = await screen.findByRole('alert')
+    expect(toast.textContent).toContain('模型操作失败：model-unavailable: session already contains images')
+    // The selection failure does not render the in-menu load strip (no Retry).
+    expect(screen.queryByRole('button', { name: '重试' })).toBeNull()
+  })
+
   it('renders no Agent-bound control for an addressed subagent session', () => {
     const load = vi.fn()
     render(<ModelSelect