frontend: alerter: improve long message support

### Current behaviour

1. #1083 added truncation with ellipses, but that means long messages can only be viewed as tooltips.
2. MAVLink [`STATUSTEXT`](https://mavlink.io/en/messages/common.html#STATUSTEXT) messages (the source of most of our alerter alerts) are currently sent immediately to the alerter, even if only partially formed

### Expected or desired behaviour

1. Ideally long messages would either wrap over multiple lines, or at least support some kind of auto-scrolling behaviour to allow reading them when they're visible (like in our [ScrollingText](https://github.com/bluerobotics/cockpit/blob/master/src/components/ScrollingText.vue) component).
2. The message definition has a limited character length, but there's a specified splitting and sequence re-combination mechanism, to allow sending longer messages, and it would be good if we could support that properly

### Prerequisites

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

Start with src/components/ScrollingText.vue to review the existing scrolling behavior, then locate the alerter code that receives MAVLink STATUSTEXT messages. Define how split messages are recombined and how long alerts are displayed. Done means long messages can be read and segmented STATUSTEXT messages appear as one complete alert.

Written by the indexing model from the issue text.