feat(web): Base UI toast region + global mutation feedback wiring (#47)

Add a module-scope Base UI toast manager bridged to the QueryClient so
every mutation can give consistent feedback. A MutationCache (extracted
into a makeQueryClient() factory for test reuse) emits a catch-all,
type-aware error toast (unless meta.suppressErrorToast) and an opt-in
success toast (meta.successMessage), reading mutation.meta + i18n.t
outside React. meta is type-checked via a react-query Register
augmentation. ToastRegion is mounted app-wide in main.tsx. Adds a toast
i18n namespace (en/sv parity) and a validated story.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/main.tsx

@@ -1,21 +1,23 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
-import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { QueryClientProvider } from "@tanstack/react-query";
 
 import { App } from "./app";
 import { ConfigProvider } from "./config/config-provider";
+import { makeQueryClient } from "./api/query-client";
+import { ToastRegion } from "./components/ui/toast";
 import "./index.css";
 import "./i18n";
 
-const queryClient = new QueryClient({
-  defaultOptions: { queries: { retry: false, refetchOnWindowFocus: false } },
-});
+const queryClient = makeQueryClient();
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
     <QueryClientProvider client={queryClient}>
       <ConfigProvider>
-        <App />
+        <ToastRegion>
+          <App />
+        </ToastRegion>
       </ConfigProvider>
     </QueryClientProvider>
   </StrictMode>,