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/routes/boards.py

@@ -32,6 +32,9 @@ def list_boards():
 @login_required
 def board_detail(board_id: int):
     board = db.get_or_404(Board, board_id)
+    # Sonoff eWeLink gateway boards have their own page
+    if board.board_type == "sonoff_ewelink":
+        return redirect(url_for("sonoff.gateway", board_id=board_id))
     # Refresh states from device
     poll_board(current_app._get_current_object(), board_id)
     board = db.session.get(Board, board_id)
@@ -54,10 +57,17 @@ def add_board():
         num_relays = int(request.form.get("num_relays", 4))
         num_inputs = int(request.form.get("num_inputs", 4))
 
-        if not name or not host:
+        # Sonoff gateway doesn't need a real host address
+        is_gateway = board_type == "sonoff_ewelink"
+        if not name or (not host and not is_gateway):
             flash("Name and host are required.", "danger")
             return render_template("boards/add.html", board_types=_board_types(), drivers=registry.all())
 
+        if is_gateway:
+            host = host or "ewelink.cloud"
+            num_relays = 0
+            num_inputs = 0
+
         board = Board(
             name=name,
             board_type=board_type,