Fix ee02 button-wakeup build: ext1 fallback for ESP32-S3

esp_sleep_enable_gpio_wakeup_on_hp_periph_powerdown() only exists on
ESP32-C6 (SOC_GPIO_SUPPORT_HP_PERIPH_PD_SLEEP_WAKEUP), so the ee02
(ESP32-S3) build failed with implicit-declaration errors in
{back,next,combo}_button.c once the epd13in3e driver's #error stopped
masking it.

Each button file now branches on that capability macro: the C6 path
(devkit/xiao) is untouched, and ESP32-S3 uses
esp_sleep_enable_ext1_wakeup_io() instead. The earlier ext1 attempt was
rejected on C6 hardware because its pull resistor didn't hold across
RTC_PERIPH power-down -- tracing the same path in ESP-IDF source shows
gpio_config()'s pull_up_en already delegates to rtc_gpio_pullup_en()
for RTC-capable pins on every non-original-ESP32 target, so the pull-up
should already survive the same power-down on S3. The _io() variant is
additive, so the three button files don't need cross-file mask
coordination. Also widens the button GPIO Kconfig range for
IDF_TARGET_ESP32S3 (0-21, matching its RTC-IO set) instead of the
C6-shaped 0-7.

Verified: ee02, devkit, and xiao all build clean end-to-end locally
(native ESP-IDF v6.0, no Docker in this sandbox). NOT verified: whether
this actually avoids the spurious-instant-wakeup bug on real EE02
hardware -- that failure mode was only ever confirmed empirically, not
root-caused in a way a compile can check. continue-on-error stays on
in CI's ee02 build step until that's confirmed.

firmware/main/back_button.c

@@ -3,6 +3,7 @@
 #include "driver/gpio.h"
 #include "esp_log.h"
 #include "esp_sleep.h"
+#include "soc/soc_caps.h"
 
 #include "freertos/FreeRTOS.h"
 #include "freertos/task.h"
@@ -35,10 +36,17 @@ void back_button_init(void)
     };
     gpio_config(&io_conf);
 
+#if SOC_GPIO_SUPPORT_HP_PERIPH_PD_SLEEP_WAKEUP
     /* See next_button.c for why this API (not ext1) -- it manages the
      * pull resistor across the sleep transition itself, so the pin
      * doesn't float and wake the device spuriously. */
     esp_sleep_enable_gpio_wakeup_on_hp_periph_powerdown(1ULL << BACK_BUTTON_GPIO, ESP_GPIO_WAKEUP_GPIO_LOW);
+#else
+    /* See next_button.c for why ext1 is safe here on targets without the
+     * API above (e.g. ESP32-S3), and why _io() needs no cross-file mask
+     * coordination. */
+    ESP_ERROR_CHECK(esp_sleep_enable_ext1_wakeup_io(1ULL << BACK_BUTTON_GPIO, ESP_EXT1_WAKEUP_ANY_LOW));
+#endif
 }
 
 back_button_result_t back_button_check(void)
@@ -49,7 +57,11 @@ back_button_result_t back_button_check(void)
      * status register is latched at the moment of waking and isn't
      * cleared until the next sleep entry, so it reliably reflects a tap
      * regardless of how quickly it was released. */
+#if SOC_GPIO_SUPPORT_HP_PERIPH_PD_SLEEP_WAKEUP
     bool caused_wake = esp_sleep_get_gpio_wakeup_status() & (1ULL << BACK_BUTTON_GPIO);
+#else
+    bool caused_wake = esp_sleep_get_ext1_wakeup_status() & (1ULL << BACK_BUTTON_GPIO);
+#endif
 
     if (!caused_wake) {
         /* Not a GPIO-wakeup-from-this-pin boot (normal timer wake, or a