fix: warn of radio pins that can stop the board booting (#34)

GPIO12 is a strapping pin on the original ESP32: high at reset, it powers
the flash at 1.8 V. A CC1101 can hold GDO2 high at power-up, so a board
with RX on 12 bootloops after a power cut until EN is pressed. It was the
default RX pin.

- RX defaults to GPIO35 on the original ESP32. Saved settings keep their
  pins.
- The log warns each time the radio starts, and the radio settings show a
  warning, while TX, RX or MISO is on a pin whose level at reset can stop
  the chip booting (src/BootPins.h, data-src/js/pins.js).

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

test/test_bootpins_s3/test_bootpins_s3.cpp

@@ -0,0 +1,38 @@
+#include <unity.h>
+#include <string.h>
+// As on the ESP32-S3, where GPIO45 picks the flash voltage.
+#define CONFIG_IDF_TARGET_ESP32S3 1
+#include "BootPins.h"
+
+void setUp() {}
+void tearDown() {}
+
+static void test_gpio45_picks_the_flash_voltage() {
+  const char *effect = bootPinEffect(45);
+  TEST_ASSERT_NOT_NULL(effect);
+  TEST_ASSERT_NOT_NULL(strstr(effect, "1.8 V"));
+}
+
+static void test_gpio0_picks_the_boot_mode() {
+  TEST_ASSERT_NOT_NULL(bootPinEffect(0));
+}
+
+static void test_gpio12_is_safe() {
+  // 12 is the S3's default SCK.
+  TEST_ASSERT_NULL(bootPinEffect(12));
+}
+
+static void test_default_radio_pins_are_safe() {
+  // transceiver_config_t::load()'s defaults for the S3.
+  const uint8_t pins[] = {15, 14, 11, 13, 12, 10};
+  for(uint8_t pin : pins) TEST_ASSERT_NULL_MESSAGE(bootPinEffect(pin), "default radio pin");
+}
+
+int main(int, char **) {
+  UNITY_BEGIN();
+  RUN_TEST(test_gpio45_picks_the_flash_voltage);
+  RUN_TEST(test_gpio0_picks_the_boot_mode);
+  RUN_TEST(test_gpio12_is_safe);
+  RUN_TEST(test_default_radio_pins_are_safe);
+  return UNITY_END();
+}