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/_ui/artist-modal.tsx

@@ -102,6 +102,8 @@ export function ArtistModal({
     if (res.ok || res.status === 409) {
       setFollowed(true);
       toast(`Following ${name}`);
+    } else {
+      toast(`Couldn't follow ${name}`);
     }
   }
 
@@ -122,6 +124,8 @@ export function ArtistModal({
     if (res.ok) {
       setWanted((s) => new Set(s).add(r.rgMbid));
       toast(`Added ${r.album}`);
+    } else {
+      toast(`Couldn't add ${r.album}`);
     }
   }