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/wanted/wanted-client.tsx

@@ -60,8 +60,8 @@ export function WantedClient() {
   }
 
   async function searchNow(w: Wanted) {
-    await fetch(`/api/releases/${w.id}/search`, { method: "POST" });
-    toast(`Searching for ${w.album}`);
+    const res = await fetch(`/api/releases/${w.id}/search`, { method: "POST" }).catch(() => null);
+    toast(res?.ok ? `Searching for ${w.album}` : `Couldn't search for ${w.album}`);
     refresh();
   }