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/pb/uart_messages.pb.go

@@ -2526,7 +2526,7 @@ func (x *EspNowEchoPingResponse) GetEspRttUs() uint32 {
 }
 
 // Host → master: LED ring on master (client_id=0) and/or slaves via ESP-NOW.
-// mode: 0=clear, 1=progress (0–100 %), 2=digit (0–10), 3=blink, 4=find-me, 5=all LEDs solid color.
+// mode: 0=clear, 1=progress (0–100 %), 2=digit (0–10), 3=blink, 4=find-me, 5=all LEDs solid color, 6=battery-low.
 type LedRingProgressRequest struct {
 	state protoimpl.MessageState `protogen:"open.v1"`
 	Mode  uint32                 `protobuf:"varint,1,opt,name=mode,proto3" json:"mode,omitempty"`