fix: replace raw array access with data_get() to prevent Undefined array key

- Migrate $protocol_settings['key'] to data_get($protocol_settings, 'key') across General, SingBox, Shadowrocket, Surfboard, QuantumultX
- Prevents PHP 8 Undefined array key fatal errors when optional protocol_settings fields are missing
- Same class of bug that caused #735

app/Protocols/QuantumultX.php

@@ -46,7 +46,7 @@ class QuantumultX extends AbstractProtocol
         $addr = Helper::wrapIPv6($server['host']);
         $config = [
             "shadowsocks={$addr}:{$server['port']}",
-            "method={$protocol_settings['cipher']}",
+            "method=" . data_get($protocol_settings, 'cipher'),
             "password={$password}",
         ];