battery: send alerts on channel "testowy" (index 2), not the pinned send channel

Owner request. BatteryAlertService reads new env BATTERY_ALERT_CHANNEL
(default 2) at construction and passes it through MeshService.sendText,
which gains an optional trailing channel argument defaulting to the
pinned MESH_SEND_CHANNEL — every other sender is untouched. Threshold,
cooldown, merged one-message-per-cycle format and @mentions unchanged.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mesh-bot",
-  "version": "0.13.0",
+  "version": "0.13.1",
   "description": "",
   "author": "",
   "private": true,