fix: swap restored radio config under the radio lock

Restoring a backup wrote the transceiver config in place from the async
task while the radio task could be transmitting with it.

Read the record into a copy and swap it in through the new
Transceiver::setConfig, which Transceiver::fromJSON now uses as well.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Somfy.h

@@ -559,6 +559,7 @@ class Transceiver {
     bool printBuffer = false;
     //bool toJSON(JsonObject& obj);
     bool fromJSON(JsonObject& obj);
+    void setConfig(const transceiver_config_t &cfg);
     bool save();
     bool begin();
     void loop();