frontend: alerts: support speech volume control

[ES-Alexander]

Current behaviour

@Williangalvani reckons TTS voice alerts are loud, which can be annoying. That's especially the case if an existing category includes a desirable alert but also some more frequent ones you don't care about at the time.

Expected or desired behaviour

1. Reduce superfluous alerts (this is already an ongoing effort)
2. Provide a volume controller for the TTS, using the relevant API

Prerequisites

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