Accessibility: Update the label on the composer to ensure form elements are visually determinable (9.3.3.2)

### Steps to reproduce

We're trying to be more accessible.

A report has noted that we do not follow: "Update the label on the composer to ensure form elements are visually determinable (9.3.3.2)"

In a previous ticket ([internal](https://github.com/element-hq/wat-internal/issues/230)) design were not comfortable meeting the strict requirements. We may have to do so in the future - we must research it first (other products do not adhere to this rule, or they have found a way round it)

### Outcome

N/A

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No