Blank screen at launch after the homeserver expires the sliding sync connection (400 M_UNKNOWN_POS)

### Steps to reproduce

1. Sign in to Element X Android and use it normally.
2. Leave the app unopened for more than 7 days. On several Android OEM builds (Xiaomi, realme, TECNO) aggressive battery management freezes the app, so it never wakes up in the background even with push configured.
3. Open the app.

### Outcome

**What did you expect?**

The app notices that the homeserver no longer knows its sliding sync position, restarts the sync from scratch and renders the room list.

**What happened instead?**

The app stays on a blank white screen indefinitely. The only workaround our users found is to clear app data / reinstall and sign in again.

That workaround is expensive: a fresh session restores the timeline from key backup, so every older message then carries the "The authenticity of this encrypted message can't be guaranteed on this device." shield.

**Server side**

Synapse deletes sliding sync connections that have not been used for `CONNECTION_EXPIRY = 7 days` (`synapse/storage/databases/main/sliding_sync.py`, cleanup runs hourly). After that the client's request

```
POST /_matrix/client/unstable/org.matrix.simplified_msc3575/sync?pos=&timeout=0
```

is answered with `400 M_UNKNOWN_POS` ("Unknown position"). The app does not appear to recover from that response.

**Scale on one self-hosted server** (Synapse 1.160.0, ~250 users, ~470 active sessions):

- 12 to 18 occurrences per day
- 11 distinct Android devices over 48 hours
- zero iOS devices, although iOS is roughly a third of the sessions here
- affected versions range from 26.05.1 to 26.09.1, so upgrading does not help

### Your phone model

Xiaomi (23028RNCAG, 2409BRN2CY, 23053RN02Y, 22120RN86G, M2006C3MNG, 24117RN76O), realme RMX3890, TECNO CL7, Samsung SM-A065F

### Operating system version

Android 10 through 16

### Application version and app store

Element X Android 26.05.1 to 26.09.1 (Google Play). Every observed version is affected, including the newest.

### Homeserver

Self-hosted Synapse 1.160.0 with Matrix Authentication Service

### Will you send logs?

No

### Will you submit a Pull Request?

No

Use the reproduction steps to trigger a stale sliding-sync position, then inspect the Android handling of the sync POST response for 400 M_UNKNOWN_POS. The server-side expiry behavior is described in synapse/storage/databases/main/sliding_sync.py. Done means the app recovers from the expired position and renders the room list without clearing app data.

Written by the indexing model from the issue text.