bug: notifications popup on boot and spam when the backend is offline

### Bug description

Notifications popup during boot saying that services are broken. If the backend is offline we get several error notifications instead of a single one. The messages are also not clear.

We should not popup on boot, not spam when the backend is down, maybe aggregate related notifications, and improve the texts.

### Steps to reproduce

1. Boot BlueOS and wait the frontend to load before the backend is ready
2. Stop the backend with the frontend still open

### Primary pain point(s)

Boot looks like something is wrong. Offline backend floods the tray.

### Additional context

The tray already cumulates consecutive similar notifications (`notifications` store), but different services still each raise their own. Snackbar is `Alerter.vue` (`alert=true`, error timeout -1). `BackendStatusChecker` pushes `BACKEND_OFFLINE` every 3s. `pushBackError` skips network/offline errors, other pollers still notify.

Related #681 #691

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

Start with Alerter.vue, the notifications store, and BackendStatusChecker, then trace pushBackError and the other pollers that raise notifications. Reproduce boot with the backend delayed and an offline backend; done means no boot popup, one aggregated offline notification instead of repeated service errors, and clearer notification text.

Written by the indexing model from the issue text.