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>

goTool/docs/API_REST.md

@@ -96,6 +96,7 @@ Body:
 | `digit` | `digit` 0–10 |
 | `blink` | `blink_ms`, `blink_count` |
 | `find-me` | Locate pod |
+| `battery-low` | First 4 LEDs red at ~10 % for 5 s (UV indicator test) |
 
 Use `client_id` (`0` = master) or `all_clients` (+ optional `slaves_only`) for broadcast.
 

goTool/docs/API_WEBSOCKET.md

@@ -214,11 +214,12 @@ Response `tap_notify_status`:
 | `digit`        | `digit` 0–10                |
 | `blink`        | `blink_ms`, `blink_count`   |
 | `find-me`      | Locate pod                  |
+| `battery-low`  | UV indicator (4 LEDs, 5 s)  |
 
 
 Target: `client_id` (`0` = master) or `all_clients` (+ optional `slaves_only`).
 
-Response `led_ring_status` — `mode` is numeric: 0=clear, 1=progress, 2=digit, 3=blink, 4=find-me, 5=color.
+Response `led_ring_status` — `mode` is numeric: 0=clear, 1=progress, 2=digit, 3=blink, 4=find-me, 5=color, 6=battery-low.
 
 ```json
 {"type":"led_ring_status","success":true,"mode":5,"client_id":16,"slaves_updated":1}