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/battery_led.h

@@ -0,0 +1,18 @@
+//
+// Low-battery LED indicator for the Pico onboard LED.
+// Reads PowerPercent / PowerState from interrupt_in_data[52]
+// (DualSense BT 0x31 report, see USBGetStateData in utils.h).
+//
+
+#pragma once
+
+void battery_led_init(void);
+
+// Call once per main-loop iteration. Drives the LED blink while the
+// battery is low and the controller is connected; otherwise no-op.
+void battery_led_tick(void);
+
+// Call from the BT input-report callback whenever a fresh 0x31 report
+// has been copied into interrupt_in_data. Used to detect disconnection
+// via stale-report timeout.
+void battery_led_note_report(void);