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/base.html

@@ -27,7 +27,7 @@
       </li>
       <li>
         <a href="{{ url_for('boards.list_boards') }}"
-           class="nav-link text-white {% if 'boards.' in request.endpoint %}active{% endif %}">
+           class="nav-link text-white {% if 'boards.' in request.endpoint or 'sonoff.' in request.endpoint %}active{% endif %}">
           <i class="bi bi-motherboard me-2"></i>Boards
         </a>
       </li>