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)

CMakeLists.txt

@@ -40,6 +40,12 @@ if(ENABLE_VERBOSE)
 else()
     set(ENABLE_VERBOSE_VALUE 0)
 endif ()
+option(ENABLE_BATT_LED "Blink Pico LED when DualSense battery is low (<=10%)" ON)
+if (ENABLE_BATT_LED)
+        set(ENABLE_BATT_LED_VALUE 1)
+else ()
+        set(ENABLE_BATT_LED_VALUE 0)
+endif ()
 
 # Pull in Raspberry Pi Pico SDK (must be before project)
 include(pico_sdk_import.cmake)
@@ -61,6 +67,10 @@ add_executable(ds5-bridge
         src/cmd.cpp
 )
 
+if (ENABLE_BATT_LED)
+        target_sources(ds5-bridge PRIVATE src/battery_led.cpp)
+endif ()
+
 # Cockos WDLResampler
 add_library(wdl_resampler STATIC
         lib/WDL/WDL/resample.cpp
@@ -88,6 +98,7 @@ target_compile_definitions(ds5-bridge PRIVATE
         PICO_STDIO_USB_ENABLE_RESET_VIA_BAUD_RATE=0
         PICO_STDIO_USB_ENABLE_RESET_VIA_VENDOR_INTERFACE=0
         ENABLE_VERBOSE=${ENABLE_VERBOSE_VALUE}
+        ENABLE_BATT_LED=${ENABLE_BATT_LED_VALUE}
 )
 
 pico_set_program_name(ds5-bridge "ds5-bridge")