Warn when the server is unavailable rather than just saying "synchronizing"

### Steps to reproduce

1. Block your server somehow (network failure; server offline)
2. Fg the app
3. Watch a neverending "Synchronizing" chip and timeline spinner
4. Wonder why your app won't sync

### Outcome

#### What did you expect?

"Offline" or "Server unavailable" or "Can't connect" or similar.

#### What happened instead?

Neverending "Synchronizing" spinner, implying that the server is accessible but is taking a long time to sync.

We should be able to distinguish the two (e.g. Threema distinguishes red & orange connectivity state) to inform the user as to what's going on.

### Your phone model

_No response_

### Operating system version

_No response_

### Application version and app store

_No response_

### Homeserver

_No response_

### Will you send logs?

No

### Are you willing to provide a PR?

No