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/test_bootpins.cpp

@@ -0,0 +1,49 @@
+#include <unity.h>
+#include <string.h>
+// No target defined: the original ESP32.
+#include "BootPins.h"
+
+void setUp() {}
+void tearDown() {}
+
+static void test_gpio12_picks_the_flash_voltage() {
+  // Issue #34: a CC1101 holding GDO2 high on GPIO12 at power-up.
+  const char *effect = bootPinEffect(12);
+  TEST_ASSERT_NOT_NULL(effect);
+  TEST_ASSERT_NOT_NULL(strstr(effect, "1.8 V"));
+}
+
+static void test_gpio0_picks_the_boot_mode() {
+  const char *effect = bootPinEffect(0);
+  TEST_ASSERT_NOT_NULL(effect);
+  TEST_ASSERT_NOT_NULL(strstr(effect, "download mode"));
+}
+
+static void test_harmless_strapping_pins_are_not_listed() {
+  // 2 only matters for the download mode, 5 and 15 set the SDIO timing and
+  // the boot log.  5 is the default CSN.
+  TEST_ASSERT_NULL(bootPinEffect(2));
+  TEST_ASSERT_NULL(bootPinEffect(5));
+  TEST_ASSERT_NULL(bootPinEffect(15));
+}
+
+static void test_default_radio_pins_are_safe() {
+  // transceiver_config_t::load()'s defaults for the original ESP32.
+  const uint8_t pins[] = {13, 35, 23, 19, 18, 5};
+  for(uint8_t pin : pins) TEST_ASSERT_NULL_MESSAGE(bootPinEffect(pin), "default radio pin");
+}
+
+static void test_other_chips_boot_pins_are_safe_here() {
+  TEST_ASSERT_NULL(bootPinEffect(9));
+  TEST_ASSERT_NULL(bootPinEffect(45));
+}
+
+int main(int, char **) {
+  UNITY_BEGIN();
+  RUN_TEST(test_gpio12_picks_the_flash_voltage);
+  RUN_TEST(test_gpio0_picks_the_boot_mode);
+  RUN_TEST(test_harmless_strapping_pins_are_not_listed);
+  RUN_TEST(test_default_radio_pins_are_safe);
+  RUN_TEST(test_other_chips_boot_pins_are_safe_here);
+  return UNITY_END();
+}