Bluetooth Off alert is critical and immediate; add a grace period before alarming #2633

Summary

When Bluetooth is switched off, Loop immediately issues the "Bluetooth Off" alert as a critical alert, so it breaks through silent mode and Focus. Users who turn Bluetooth off briefly on purpose get an alarm the moment they do it. Examples: in a meeting, at a religious service, or at night next to a sleeping partner.

Current behavior

AlertManager.onBluetoothPoweredOff() (Loop/Managers/Alerts/AlertManager.swift) issues bluetoothPoweredOff with trigger: .immediate and interruptionLevel: .critical as soon as bluetoothDidUpdateState reports .poweredOff. onBluetoothPoweredOn() retracts it. The behavior is the same on dev and next-dev.

Suggested behavior

Hold the alert for a grace period after Bluetooth goes off, and retract it if Bluetooth comes back first. Options:

• trigger: .delayed(interval:) with a few minutes' grace. The existing retract on power-on already cancels a pending delayed alert.
• A non-critical alert first, escalating to critical only if Bluetooth stays off.
• Possibly tie the timing to how long Loop can go without CGM or pump communication before it matters.

The permission-denied path (onBluetoothPermissionDenied) can stay immediate.