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/pod_settings.c

@@ -7,6 +7,7 @@
 static const char *TAG = "[SETTINGS]";
 static const char *NS = "powerpod";
 static const char *KEY_ACCEL_DZ = "accel_dz";
+static const char *KEY_UV_LATCH = "uv_latch";
 
 #define ACCEL_DEADZONE_MAX 4095u
 
@@ -108,3 +109,53 @@ void pod_settings_apply_accel_deadzone(void) {
     bma456_set_accel_deadzone(deadzone);
   }
 }
+
+bool pod_settings_is_uv_latched(void) {
+  if (!s_nvs_ready) {
+    return false;
+  }
+
+  nvs_handle_t handle;
+  esp_err_t err = nvs_open(NS, NVS_READONLY, &handle);
+  if (err != ESP_OK) {
+    return false;
+  }
+
+  uint8_t value = 0;
+  err = nvs_get_u8(handle, KEY_UV_LATCH, &value);
+  nvs_close(handle);
+
+  return err == ESP_OK && value != 0;
+}
+
+esp_err_t pod_settings_set_uv_latched(bool latched) {
+  esp_err_t err = ensure_nvs();
+  if (err != ESP_OK) {
+    return err;
+  }
+
+  nvs_handle_t handle;
+  err = nvs_open(NS, NVS_READWRITE, &handle);
+  if (err != ESP_OK) {
+    ESP_LOGE(TAG, "nvs_open failed: %s", esp_err_to_name(err));
+    return err;
+  }
+
+  err = nvs_set_u8(handle, KEY_UV_LATCH, latched ? 1u : 0u);
+  if (err == ESP_OK) {
+    err = nvs_commit(handle);
+  }
+  nvs_close(handle);
+
+  if (err != ESP_OK) {
+    ESP_LOGE(TAG, "save uv_latch failed: %s", esp_err_to_name(err));
+    return err;
+  }
+
+  ESP_LOGI(TAG, "UV latch %s", latched ? "set" : "cleared");
+  return ESP_OK;
+}
+
+esp_err_t pod_settings_clear_uv_latched(void) {
+  return pod_settings_set_uv_latched(false);
+}