fix(web): surface a toast on failed mutations (#14)

Several client mutations either ignored a non-ok response (silent no-op) or
toasted success unconditionally even when the request failed. Sweep them so every
mutation gives feedback:

- artists: toggle auto-monitor, unfollow → error toast on failure.
- discography + wanted: toggle monitor / search-now → success or error toast.
- discover suggestion follow/want/dismiss → error toast (was toasting success
  even on failure).
- The Floor: queue request + retry → error toast on failure.
- artist-modal + preview follow/want, Last.fm want → error toast on failure
  (closes the deferred "want() shows no toast on non-ok" item).

web 153 tests, tsc + build clean.

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

web/src/app/artists/artists-client.tsx

@@ -95,16 +95,18 @@ export function ArtistsClient() {
   }
 
   async function toggleAuto(a: Artist) {
-    await fetch(`/api/artists/${a.id}`, {
+    const res = await fetch(`/api/artists/${a.id}`, {
       method: "PATCH",
       headers: { "content-type": "application/json" },
       body: JSON.stringify({ autoMonitorFuture: !a.autoMonitorFuture }),
-    });
+    }).catch(() => null);
+    if (!res?.ok) toast(`Couldn't update ${a.name}`);
     refresh();
   }
 
   async function unfollow(a: Artist) {
-    await fetch(`/api/artists/${a.id}`, { method: "DELETE" });
+    const res = await fetch(`/api/artists/${a.id}`, { method: "DELETE" }).catch(() => null);
+    if (!res?.ok) toast(`Couldn't unfollow ${a.name}`);
     refresh();
   }