feat(web): move Hidden tracks out of main nav, surface from Settings

Hidden tracks (M5b quarantine) is a low-frequency surface — it doesn't
warrant a permanent slot in the main nav. Moves the link under a new
'Library' section card on /settings alongside ListenBrainz config.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/routes/settings/+page.svelte

@@ -97,4 +97,14 @@
       </div>
     {/if}
   </section>
+
+  <section class="space-y-3 rounded border border-border bg-surface p-4">
+    <h2 class="text-lg font-semibold">Library</h2>
+    <ul class="space-y-2 text-sm">
+      <li>
+        <a href="/library/hidden" class="text-accent hover:underline">Hidden tracks</a>
+        <span class="text-text-secondary"> — tracks you've flagged as bad rips, wrong tags, or otherwise unplayable.</span>
+      </li>
+    </ul>
+  </section>
 </div>