fix(web/test): mount ToastHost in vitest setup so toast assertions resolve

W-T3 (617477b) consolidated toast rendering into a single <ToastHost />
mounted in +layout.svelte, but page-level tests render components without
the layout, so screen.getByTestId('toast') and role=status queries broke.
Mount ToastHost in beforeEach (auto-cleaned by svelteTesting()) and reset
the store with clearToast() in afterEach so toasts can't leak between
tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/vitest.setup.ts

@@ -1,4 +1,8 @@
 import '@testing-library/jest-dom/vitest';
+import { beforeEach, afterEach } from 'vitest';
+import { render } from '@testing-library/svelte';
+import ToastHost from './src/lib/components/ToastHost.svelte';
+import { clearToast } from './src/lib/stores/toast.svelte';
 
 // Node 25 ships a partial localStorage global that lacks getItem/setItem/clear
 // unless launched with --localstorage-file. jsdom sees Node's global and skips
@@ -21,3 +25,20 @@ if (typeof window !== 'undefined') {
   Object.defineProperty(window, 'localStorage', { configurable: true, value: memLocal });
   Object.defineProperty(window, 'sessionStorage', { configurable: true, value: memSession });
 }
+
+// W-T3 moved toast rendering out of per-page markup into a single
+// <ToastHost /> mounted in +layout.svelte. Tests render individual pages
+// without the layout, so we mount ToastHost here so `pushToast()` calls
+// from page code surface in the DOM the same way they do in production.
+// `render()` from @testing-library/svelte registers auto-cleanup via the
+// svelteTesting() vite plugin, so the host gets unmounted between tests
+// alongside whatever component the test itself rendered. clearToast()
+// resets the module-scope store state so a stale toast from one test
+// can't leak into another.
+beforeEach(() => {
+  render(ToastHost);
+});
+
+afterEach(() => {
+  clearToast();
+});