feat: low-battery LED indicator (ENABLE_BATT_LED, default ON)

Blink the Pico onboard LED at 1 Hz when the connected DualSense
reports PowerPercent <= 1 (i.e. <= 10%) and PowerState == Discharging.
Source data is byte 52 of the BT 0x31 input report, already copied
into interrupt_in_data; no new BT parsing required.

The new module owns the LED only while it is actively blinking; it
detects controller disconnection via stale-report timeout and steps
out, so bt.cpp's existing connect/disconnect LED handling stays in
charge in all other states. Honors disable_pico_led.

Gated by -DENABLE_BATT_LED=ON (default). With the option off, the
source file is not compiled and behavior is identical to upstream.
CI gains a compile-only check for the OFF flavor.

(cherry picked from commit 2f8ea73c9fb695e24e7cc3329db7cb925e82e1c9)

src/main.cpp

@@ -17,6 +17,9 @@
 #endif
 #include "config.h"
 #include "cmd.h"
+#if ENABLE_BATT_LED
+#include "battery_led.h"
+#endif
 
 // Pico SDK speciifically for waiting on conditions
 #include "pico/critical_section.h"
@@ -85,6 +88,9 @@ void on_bt_data(CHANNEL_TYPE channel, uint8_t *data, uint16_t len) {
 
         if (get_config().polling_rate_mode != 2) {
             memcpy(interrupt_in_data, data + 3, 63);
+#if ENABLE_BATT_LED
+            battery_led_note_report();
+#endif
             return;
         }
 
@@ -98,6 +104,9 @@ void on_bt_data(CHANNEL_TYPE channel, uint8_t *data, uint16_t len) {
         memcpy(interrupt_in_data, data + 3, 63);
         report_dirty = true;
         critical_section_exit(&report_cs);
+#if ENABLE_BATT_LED
+        battery_led_note_report();
+#endif
     }
 }
 
@@ -204,6 +213,10 @@ int main() {
     }
     cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, false);
 
+#if ENABLE_BATT_LED
+    battery_led_init();
+#endif
+
 #if !ENABLE_SERIAL
     if (watchdog_caused_reboot()) {
         printf("Rebooted by Watchdog!\n");
@@ -243,5 +256,8 @@ int main() {
         tud_task();
         audio_loop();
         interrupt_loop();
+#if ENABLE_BATT_LED
+        battery_led_tick();
+#endif
     }
 }