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/api/react-query.d.ts

@@ -0,0 +1,14 @@
+import "@tanstack/react-query";
+
+declare module "@tanstack/react-query" {
+  interface Register {
+    mutationMeta: {
+      /** i18n key for a success toast (opt-in). */
+      successMessage?: string;
+      /** i18n key overriding the default error toast message. */
+      errorMessage?: string;
+      /** Skip the global error toast (the component shows the error inline). */
+      suppressErrorToast?: boolean;
+    };
+  }
+}