Ignore 0% battery readings in low-battery alerting

A 0% batteryLevel is an invalid/no-data value (no battery wired or
misconfigured node), not a genuinely critical battery, so it must not
produce a "⚠️ battery low" alert. Add a non-positive guard to
shouldAlert after the undefined check: 0 (and any non-positive) now
skips, while 1-20 still alert and >20 still doesn't. Threshold/cooldown/
batching unchanged.

Tests: 3 new specs in the check-threshold block (0% excluded from the
batch, lone 0% sends nothing, 1% still alerts). 180 -> 183.

src/battery/battery.service.spec.ts

@@ -119,6 +119,47 @@ describe('BatteryAlertService check threshold', () => {
     expect(sendText).not.toHaveBeenCalled();
   });
 
+  it('ignores a 0% reading (invalid/no battery) even though 0 <= threshold', async () => {
+    const sendText = vi.fn(async (..._args: unknown[]) => 1);
+    // 0% must not alert, but a genuinely-low 5% node in the same batch must
+    const svc = makeService([node(1, 0), node(2, 5)], sendText);
+
+    await svc.check();
+
+    expect(sendText).toHaveBeenCalledTimes(1);
+    // only the 5% node — the 0% node is excluded entirely
+    expect(sendText.mock.calls[0]).toEqual([
+      BROADCAST_ADDR,
+      '⚠️ battery low: @!00000002 5%',
+      undefined,
+      2,
+    ]);
+  });
+
+  it('ignores a lone 0% reading (no broadcast at all)', async () => {
+    const sendText = vi.fn(async (..._args: unknown[]) => 1);
+    const svc = makeService([node(1, 0)], sendText); // only an invalid 0% reading
+
+    await svc.check();
+
+    expect(sendText).not.toHaveBeenCalled();
+  });
+
+  it('still alerts a genuine 1% reading (lowest valid value)', async () => {
+    const sendText = vi.fn(async (..._args: unknown[]) => 1);
+    const svc = makeService([node(1, 1)], sendText);
+
+    await svc.check();
+
+    expect(sendText).toHaveBeenCalledTimes(1);
+    expect(sendText.mock.calls[0]).toEqual([
+      BROADCAST_ADDR,
+      '⚠️ battery low: @!00000001 1%',
+      undefined,
+      2,
+    ]);
+  });
+
   it('merges all qualifying nodes into exactly ONE broadcast with the broadcast target', async () => {
     const sendText = vi.fn(async (..._args: unknown[]) => 1);
     const svc = makeService(

src/battery/battery.service.ts

@@ -60,6 +60,7 @@ export class BatteryAlertService {
 
   private shouldAlert(node: NodeSummary, now: number): boolean {
     if (node.batteryLevel === undefined) return false; // no battery reported — skip
+    if (node.batteryLevel <= 0) return false; // 0/invalid reading — no battery wired or misconfigured node, not a real low-battery alert
     // <= on purpose: a node at exactly the threshold is already "low" (spec allowed either < or <=; tests pin this choice)
     if (!(node.batteryLevel <= this.threshold)) return false;
     const last = this.lastAlertAt.get(node.num);