Fix ESP32-S2 build: USB.h, WDT API, remove duplicate flags

- Add #include <USB.h> to all .cpp files (required for HWCDCSerial)
- Use ESP-IDF v5.x esp_task_wdt_config_t struct instead of removed API
- Remove duplicate ARDUINO_USB_MODE/CDC flags from platformio.ini
  (board definition already provides these)

firmware/platformio.ini

@@ -9,6 +9,3 @@ monitor_speed = 115200
 board_build.filesystem = littlefs
 build_flags =
     -DCORE_DEBUG_LEVEL=3
-    -DBOARD_HAS_PSRAM=1
-    -DARDUINO_USB_MODE=1
-    -DARDUINO_USB_CDC_ON_BOOT=1

firmware/src/attenuator.cpp

@@ -1,4 +1,5 @@
 #include "attenuator.h"
+#include <USB.h>
 #include <soc/gpio_struct.h>
 
 Attenuator::Attenuator() : _step(0) {}

firmware/src/main.cpp

@@ -1,4 +1,5 @@
 #include <Arduino.h>
+#include <USB.h>
 #include <WiFi.h>
 #include <ESPmDNS.h>
 #include <esp_task_wdt.h>
@@ -196,8 +197,13 @@ void setup() {
     pinMode(PIN_LED, OUTPUT);
     setLEDState(LEDState::Off);
 
-    // Initialize watchdog
-    esp_task_wdt_init(WDT_TIMEOUT_S, true);
+    // Initialize watchdog (ESP-IDF v5.x API)
+    esp_task_wdt_config_t wdt_config = {
+        .timeout_ms = WDT_TIMEOUT_S * 1000,
+        .idle_core_mask = (1 << portNUM_PROCESSORS) - 1,  // All cores
+        .trigger_panic = true
+    };
+    esp_task_wdt_init(&wdt_config);
     esp_task_wdt_add(NULL);
     Serial.printf("[WDT] Initialized, timeout=%d s\n", WDT_TIMEOUT_S);
 

firmware/src/web_server.cpp

@@ -1,6 +1,7 @@
 #include "web_server.h"
 
 #include <Arduino.h>
+#include <USB.h>
 #include <WiFi.h>
 #include <ESPAsyncWebServer.h>
 #include <ArduinoJson.h>