Fix SonoffLAN signing key: hardcode pre-computed HMAC key

The previous _compute_sign_key() function indexed into a base64 string
derived from the full SonoffLAN REGIONS dict (~243 entries). Our partial
dict only produced a 7876-char a string but needed index 7872+, so the
function must use the full dict.

Solution: pre-compute the key once from the full dict and hardcode the
resulting 32-byte ASCII key. This is deterministic — the SonoffLAN
algorithm always produces the same output regardless of when it runs.

The sonoff_ewelink driver now loads cleanly alongside all other drivers.

app/templates/boards/list.html

@@ -22,7 +22,15 @@
     <tbody>
       {% for b in boards %}
       <tr>
-        <td><a href="{{ url_for('boards.board_detail', board_id=b.id) }}" class="fw-semibold text-decoration-none">{{ b.name }}</a></td>
+        <td>
+          {% if b.board_type == 'sonoff_ewelink' %}
+          <a href="{{ url_for('sonoff.gateway', board_id=b.id) }}" class="fw-semibold text-decoration-none">
+            <i class="bi bi-cloud-lightning-fill text-warning me-1"></i>{{ b.name }}
+          </a>
+          {% else %}
+          <a href="{{ url_for('boards.board_detail', board_id=b.id) }}" class="fw-semibold text-decoration-none">{{ b.name }}</a>
+          {% endif %}
+        </td>
         <td><span class="badge text-bg-secondary">{{ b.board_type }}</span></td>
         <td class="font-monospace small">{{ b.host }}:{{ b.port }}</td>
         <td>{{ b.num_relays }}</td>
@@ -36,7 +44,7 @@
           {% if b.last_seen %}{{ b.last_seen.strftime('%Y-%m-%d %H:%M') }}{% else %}—{% endif %}
         </td>
         <td>
-          <a href="{{ url_for('boards.board_detail', board_id=b.id) }}" class="btn btn-sm btn-outline-primary me-1"><i class="bi bi-eye"></i></a>
+          <a href="{% if b.board_type == 'sonoff_ewelink' %}{{ url_for('sonoff.gateway', board_id=b.id) }}{% else %}{{ url_for('boards.board_detail', board_id=b.id) }}{% endif %}" class="btn btn-sm btn-outline-primary me-1"><i class="bi bi-eye"></i></a>
           {% if current_user.is_admin() %}
           <a href="{{ url_for('boards.edit_board', board_id=b.id) }}" class="btn btn-sm btn-outline-secondary me-1"><i class="bi bi-pencil"></i></a>
           <form method="POST" action="{{ url_for('boards.delete_board', board_id=b.id) }}" class="d-inline"