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/autotest/runner_test.go

@@ -12,6 +12,7 @@ func TestLedRingModeValue(t *testing.T) {
 		{"digit", 2},
 		{"blink", 3},
 		{"find-me", 4},
+		{"battery-low", 6},
 	}
 	for _, tc := range tests {
 		got, err := ledRingModeValue(tc.mode)