Refactor Config_Web to use heap-allocated JSON document and add stack usage check script

src/Config_Web.cpp

@@ -9,6 +9,7 @@ namespace
 	const size_t kConfigJsonCapacity = 4096;
 	const byte kDnsPort = 53;
 	const unsigned long kWifiReconnectIntervalMs = 10000;
+	using ConfigJsonDocument = StaticJsonDocument<kConfigJsonCapacity>;
 
 	WebServer server(80);
 	DNSServer dnsServer;
@@ -17,6 +18,14 @@ namespace
 	bool staReconnectEnabled = false;
 	bool staConnectionKnown = false;
 	unsigned long lastStaReconnectAttemptMs = 0;
+	ConfigJsonDocument g_configJsonScratch;
+
+	ConfigJsonDocument &configJsonScratch()
+	{
+		// Keep the large config JSON buffer off ESP32 Arduino's 8 KB loopTask stack.
+		g_configJsonScratch.clear();
+		return g_configJsonScratch;
+	}
 
 	void redirectToRoot()
 	{
@@ -371,7 +380,7 @@ bool loadConfigFromFile()
 		return saveConfigToFile();
 	}
 
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	DeserializationError err = deserializeJson(doc, file);
 	file.close();
 	if (err)
@@ -424,7 +433,7 @@ bool saveConfigToFile()
 		return false;
 	}
 
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	JsonArray params = doc.createNestedArray("parameters");
 	for (size_t i = 0; i < configParameterCount; ++i)
 	{
@@ -461,7 +470,7 @@ bool saveConfigToFile()
 
 String buildConfigJson()
 {
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	JsonArray params = doc.createNestedArray("parameters");
 	for (size_t i = 0; i < configParameterCount; ++i)
 	{
@@ -497,7 +506,7 @@ String buildConfigJson()
 
 bool applyConfigJson(const String &payload, String &errorMessage)
 {
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	DeserializationError err = deserializeJson(doc, payload);
 	if (err)
 	{