feat(11-04): add client name links in printer_list.html and client_list.html

- printer_list.html: extract group_client_id from first printer in group;
  wrap client name in <a href="/clients/{id}"> when id present; Unassigned
  group renders as plain text (no dead link)
- client_list.html: wrap client name cell in <a href="/clients/{{ c.id }}">

imptune/templates/partials/client_list.html

@@ -12,7 +12,7 @@
     <tbody>
       {% for c in clients %}
       <tr>
-        <td>{{ c.name }}</td>
+        <td><a href="/clients/{{ c.id }}">{{ c.name }}</a></td>
         <td>{{ c.created_at.strftime('%Y-%m-%d') if c.created_at else '—' }}</td>
       </tr>
       {% endfor %}