Messages sent in thread can vanish entirely.

### Steps to reproduce

1. Open a thread
2. Write a message in the threadpanel while it's still loading
3. Local & remote echo vanishes entirely

### Outcome

#### What did you expect?

Reliable threads

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

Element Nightly version: 2024040901 Crypto version: Rust SDK 0.7.0 (61b175b), Vodozemac 0.5.1

### Homeserver

_No response_

### Will you send logs?

No

Start by reproducing the issue in the threadpanel while the thread is still loading, then trace the message send and local/remote echo handling from that entry point. Done means a message sent during loading remains visible and is reliably delivered rather than vanishing.

Written by the indexing model from the issue text.