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/queue.tsx

@@ -76,22 +76,28 @@ export function Queue() {
   }, []);
 
   async function retry(id: string) {
-    const res = await fetch(`/api/requests/${id}/retry`, { method: "POST" });
-    if (res.ok) {
-      toast("Retrying…");
-      refresh();
+    const res = await fetch(`/api/requests/${id}/retry`, { method: "POST" }).catch(() => null);
+    if (!res?.ok) {
+      toast("Couldn't retry");
+      return;
     }
+    toast("Retrying…");
+    refresh();
   }
 
   async function submit(e: React.FormEvent) {
     e.preventDefault();
     if (!artist.trim() || !album.trim()) return;
     const label = album.trim();
-    await fetch("/api/requests", {
+    const res = await fetch("/api/requests", {
       method: "POST",
       headers: { "content-type": "application/json" },
       body: JSON.stringify({ artist, album }),
-    });
+    }).catch(() => null);
+    if (!res?.ok) {
+      toast(`Couldn't queue ${label}`);
+      return;
+    }
     toast(`Queued ${label}`);
     setArtist("");
     setAlbum("");