No UI feedback when EW is catching up on to-device msgs.

### Steps to reproduce

1. Open EW for the first time in a while (or after something has DoSed you with to-device msgs such as keyshare reqs)
2. E2EE all shows up UTD
3. Have no way of knowing that it's because it's chewing through a massive backlog of to-device traffic.

### Outcome

#### What did you expect?

Some kind of UI indication saying that it's syncing.

Ideally, we'd copy EX's semantics (although EX also suffers from https://github.com/element-hq/element-x-ios/issues/1269)

#### What happened instead?

No feedback; crypto feels flakey.

### 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