Fix dedup apply lock-hang, mawk-broken tag passes; add track delete, fix button/select CSS

confirm_and_apply no longer marks candidates confirmed before their apply
job actually succeeds (a stuck/skipped_lock run was silently vanishing
confirmed deletes without deleting anything), switches the fuzzy pass to
--apply-pairs to avoid a full rescan on every confirm, and adds a job
timeout so a hung subprocess can't hold the global pipeline lock forever.

dedup-library.sh's Pass 2-4 use gawk-only features (gensub, POSIX interval
regex) but were running under mawk (Debian's default awk) in the deployed
image, throwing silent syntax errors on every run -- switched those
invocations to gawk explicitly and added it to the Dockerfile.

Also: per-track delete button on the library list/detail pages, and two
CSS fixes (the primary button's hover gradient was getting clobbered by
the base .btn:hover rule's plain background, and the select dropdown
arrow had no background-size so it rendered oversized).

app/templates/library/index.html

@@ -8,6 +8,10 @@
   </div>
 </div>
 
+{% if request.query_params.get('deleted') %}
+<div class="notice success">Track deleted.</div>
+{% endif %}
+
 <form method="get" action="/library" class="panel" style="display:flex; gap:0.75rem; flex-wrap:wrap; align-items:flex-end; max-width:none;">
   <div class="field">
     <label for="q">Search</label>
@@ -47,7 +51,13 @@
         <td>{{ item.title }}</td>
         <td class="muted">{{ item.albumartist }}</td>
         <td><span class="badge badge-muted">{{ item.format }}</span></td>
-        <td><a href="/library/track/{{ item.id }}" class="btn btn-sm btn-ghost">Edit</a></td>
+        <td class="actions-cell">
+          <a href="/library/track/{{ item.id }}" class="btn btn-sm btn-ghost">Edit</a>
+          <form method="post" action="/library/track/{{ item.id }}/delete" class="inline"
+                onsubmit="return confirm('Delete {{ item.artist }} — {{ item.title }}? This removes the file from disk permanently.')">
+            <button type="submit" class="btn btn-sm btn-danger">Delete</button>
+          </form>
+        </td>
       </tr>
       {% else %}
       <tr class="empty-row"><td colspan="5">

app/templates/library/track_detail.html

@@ -50,4 +50,13 @@
     <button type="submit" class="btn">Retag</button>
   </form>
 </div>
+
+<h2>Danger zone</h2>
+<div class="panel" style="max-width:560px;">
+  <p class="muted" style="margin-top:0;">Removes this track from beets and deletes the file from disk. Not reversible.</p>
+  <form method="post" action="/library/track/{{ item.id }}/delete"
+        onsubmit="return confirm('Delete {{ item.artist }} — {{ item.title }}? This removes the file from disk permanently.')">
+    <button type="submit" class="btn btn-danger">Delete this track</button>
+  </form>
+</div>
 {% endblock %}