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/[id]/discography-client.tsx

@@ -4,6 +4,7 @@ import { useEffect, useMemo, useState } from "react";
 import { PageHead } from "../../_ui/page-head";
 import { CoverArt } from "../../_ui/cover-art";
 import { AlbumModal } from "../../_ui/album-modal";
+import { toast } from "../../_ui/toast";
 import { categoryOf, TAB_ORDER, type Category } from "../../_ui/categories";
 
 type Release = {
@@ -39,15 +40,17 @@ export function DiscographyClient({ id }: { id: string }) {
   }, [id]);
 
   async function toggleMonitor(r: Release) {
-    await fetch(`/api/releases/${r.id}`, {
+    const res = await fetch(`/api/releases/${r.id}`, {
       method: "PATCH",
       headers: { "content-type": "application/json" },
       body: JSON.stringify({ monitored: !r.monitored }),
-    });
+    }).catch(() => null);
+    if (!res?.ok) toast(`Couldn't update ${r.album}`);
     refresh();
   }
   async function searchNow(r: Release) {
-    await fetch(`/api/releases/${r.id}/search`, { method: "POST" });
+    const res = await fetch(`/api/releases/${r.id}/search`, { method: "POST" }).catch(() => null);
+    toast(res?.ok ? `Searching for ${r.album}` : `Couldn't search for ${r.album}`);
     refresh();
   }