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/CMakeLists.txt

@@ -42,6 +42,7 @@ idf_component_register(
   "esp_now_proto.c"
   "bosch456.c"
   "board_input.c"
+  "battery_uv.c"
   "pod_settings.c"
   "proto/uart_messages.pb.c"
   "proto/esp_now_messages.pb.c"
@@ -68,3 +69,5 @@ idf_component_register(
 
 target_compile_definitions(${COMPONENT_LIB}
                            PRIVATE "POWERPOD_GIT_HASH=\"${POWERPOD_GIT_HASH}\"")
+# Optional: disable software UV protection
+# target_compile_definitions(${COMPONENT_LIB} PRIVATE POWERPOD_BATTERY_UV_ENABLE=0)