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.

src/mesh/mesh.service.ts

@@ -490,22 +490,23 @@ export class MeshService implements OnModuleInit, OnModuleDestroy {
     return [...this.telemetryHistory].reverse();
   }
 
-  /** Send `text` to node `to` on the pinned test channel; returns the packet id. Optional `replyId` marks the message as a reply to an earlier packet (the recipient app renders it as a threaded quote). Throws ServiceUnavailable when the bridge is down. */
-  async sendText(to: number, text: string, replyId?: number): Promise<number> {
+  /** Send `text` to node `to`; returns the packet id. Rides the pinned test channel unless an explicit `channel` index is given (e.g. battery alerts on "testowy"). Optional `replyId` marks the message as a reply to an earlier packet (the recipient app renders it as a threaded quote). Throws ServiceUnavailable when the bridge is down. */
+  async sendText(
+    to: number,
+    text: string,
+    replyId?: number,
+    channel?: number,
+  ): Promise<number> {
     if (!this.connected || !this.socket)
       throw new ServiceUnavailableException('bridge not connected');
-    const { bytes, id } = await buildTextToRadio(
-      to,
-      text,
-      this.sendChannel,
-      replyId,
-    );
+    const ch = channel ?? this.sendChannel;
+    const { bytes, id } = await buildTextToRadio(to, text, ch, replyId);
     const rec: MessageRecord = {
       id,
       dir: 'tx',
       to,
       text,
-      channel: this.sendChannel,
+      channel: ch,
       at: Date.now(),
       status: 'pending',
     };