initial sync on restart

### Steps to reproduce

1. Run Element (desktop)
2. Restart
3. Wait for ages because homeserver is unhappy
4. Open network inspector
5. See that it is doing some sync requests with `filter=7&timeout=0&set_presence=online` and no `since` parameter

### Outcome

#### What did you expect?

That it would not do an initial sync, but reuse the last `since` token that it received before restarting.

#### What happened instead?

### Operating system

Debian Linux stable

### Application version

Nightly 2024082701, Rust SDK 0.7.1 (c8c9d15), Vodozemac 0.6.0

### How did you install the app?

.deb from packages.element.io

### Homeserver

synapse.uhoreg.ca, Synapse 1.113.0 from packages.matrix.org

### Will you send logs?

Yes

No source file or test is named in the report. Start by tracing the restart flow and the sync requests described in the network inspector, then check how the last `since` token is handled across restart. Done means restart no longer triggers an initial sync and reuses the previously received token.

Written by the indexing model from the issue text.