Fix repeated low battery notifications

[SpencerCurtis]

What changed?

• Add cooldown interval between notifications using lastLowBatteryNotification timestamp on NodeInfoEntity.
• Respects user-configurable interval from Settings (Settings → Meshtastic → Low Battery Interval).

Why did it change?

Low battery notifications were being sent repeatedly every few seconds while battery remained below threshold, flooding users with duplicate alerts. Mentioned in issue 1539

How is this tested?

• Connect to a node with battery level < 4%
• Verify only one notification is sent
• Wait for configured interval to elapse and verify subsequent notification is allowed
• Change interval in Settings app and confirm new interval is respected

Screenshots/Videos (when applicable)

Checklist

• My code adheres to the project's coding and style guidelines.
• I have conducted a self-review of my code.
• I have commented my code, particularly in complex areas.
• I have verified whether these changes require an update to existing documentation or if new documentation is needed, and created an issue in the docs repo if applicable.
• I have tested the change to ensure that it works as intended.

[CLAassistant]

All committers have signed the CLA.

[SpencerCurtis]

Ah, the second screenshot is slightly wrong. I removed the 1 minute interval

[garthvh]

Not sure why you were seeing notifications every second, seems like it might be a hardware issue. The device metrics are sent every minute and iOS 26 groups notifications together.

[SpencerCurtis]

The issue wasn't made by me, but I was able to reproduce it locally.

[garthvh]

The data used to create the notifications comes in every minute, not sure how it would come in any period shorter than that.

[SpencerCurtis]

I'm happy to close the PR if you'd prefer. Either way!