frontend: Add desktop notification for critical alerts on Electron Cockpit version

### Current behaviour

When running on Electron, Cockpit does not have the voice synthesizer as on the web version.

### Expected or desired behaviour

Adding desktop notifications for critical warnings is an efficient way of communicating urgent messages to the user.

### Prerequisites

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

Start by tracing the Electron path and the handling of critical warnings, comparing it with the web version's voice-synthesizer behavior. Done means that critical warnings produce desktop notifications in the Electron version and that the behavior is verified in the relevant Electron workflow.

Written by the indexing model from the issue text.