Add software LiPo UV protection with energy-save boot.

Detect under-voltage on GPIO1/11, persist UV state in NVS, enter a minimal
power mode without ESP-NOW, and recover after charge; expose LED mode 6
battery-low for host testing via goTool.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/led_ring.h

@@ -7,6 +7,10 @@
 #define LED_RING_DEFAULT_INTENSITY 13
 /** Full brightness for find-me and similar alerts. */
 #define LED_RING_FULL_INTENSITY 255
+/** ~10 % brightness for battery-low indicator. */
+#define LED_RING_BATTERY_LOW_INTENSITY 26
+#define LED_RING_BATTERY_LOW_LEDS 4
+#define LED_RING_BATTERY_LOW_HOLD_MS 5000
 
 typedef enum {
   LED_CMD_CLEAR,
@@ -14,7 +18,8 @@ typedef enum {
   LED_CMD_SET_COLOR,
   LED_CMD_PROGRESS,
   LED_CMD_BLINK,
-  LED_CMD_FIND_ME
+  LED_CMD_FIND_ME,
+  LED_CMD_BATTERY_LOW
 } led_mode_t;
 
 typedef struct {
@@ -45,4 +50,7 @@ void led_ring_ota_failed(void);
 /** Red / green / blue: 3 blinks each at full intensity. */
 void led_ring_find_me(void);
 
+/** First 4 LEDs red at ~10 % for 5 s, then off (blocking in LED task). */
+void led_ring_show_battery_low(void);
+
 #endif