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/cmd_led_ring.go

@@ -9,7 +9,7 @@ import (
 
 func runLedRing(sp *serialPort, args []string) error {
 	fs := flag.NewFlagSet("led-ring", flag.ExitOnError)
-	mode := fs.String("mode", "progress", "clear, color, progress, digit, blink, or find-me")
+	mode := fs.String("mode", "progress", "clear, color, progress, digit, blink, find-me, or battery-low")
 	clientID := fs.Uint("client", 0, "0=master ring, >0=slave via ESP-NOW")
 	allClients := fs.Bool("all", false, "broadcast to all slaves")
 	slavesOnly := fs.Bool("slaves-only", false, "with -all: do not change master ring")