Add error checking for fwrite operations per code review

Co-authored-by: dorkmo <[email protected]>

Author: Copilot

TankAlarm-112025-Client-BluesOpta/TankAlarm-112025-Client-BluesOpta.ino

@@ -784,8 +784,12 @@ static bool saveConfigToFlash(const ClientConfig &cfg) {
       return false;
     }
 
-    fwrite(jsonStr.c_str(), 1, jsonStr.length(), file);
+    size_t written = fwrite(jsonStr.c_str(), 1, jsonStr.length(), file);
     fclose(file);
+    if (written != jsonStr.length()) {
+      Serial.println(F("Failed to write config (incomplete)"));
+      return false;
+    }
     return true;
   #else
     File file = LittleFS.open(CLIENT_CONFIG_PATH, "w");

TankAlarm-112025-Server-BluesOpta/TankAlarm-112025-Server-BluesOpta.ino

@@ -3013,8 +3013,12 @@ static bool saveConfig(const ServerConfig &cfg) {
       return false;
     }
 
-    fwrite(jsonStr.c_str(), 1, jsonStr.length(), file);
+    size_t written = fwrite(jsonStr.c_str(), 1, jsonStr.length(), file);
     fclose(file);
+    if (written != jsonStr.length()) {
+      Serial.println(F("Failed to write server config (incomplete)"));
+      return false;
+    }
     return true;
   #else
     File file = LittleFS.open(SERVER_CONFIG_PATH, "w");