Room UI is confusing when starting a DM fails

### Steps to reproduce

1. Start a DM
2. DM creation fails
3. Notice the confusing UI state

### Outcome

#### What did you expect?

The UI looks like a mix of being in the room and not being in the room. I'd expect that the prompt to send my first message only appears when I can actually type messages. Also the message that encryption is enabled feels irritating when I'm supposedly not in the room yet.

#### What happened instead?

The screen says "encryption enabled" and "send your first message to invite $user" but at the bottom there's the error "Could not start chat [retry]".

### Operating system

macOS

### Application version

Element Nightly version: 2026061501 Crypto version: Rust SDK 0.18.0 (6fe03c1), Vodozemac 0.10.0

### How did you install the app?

element.io

### Homeserver

matrix.org

### Will you send logs?

Yes

Reproduce the failed DM flow in Element Nightly on macOS, starting with the room UI state shown alongside “Could not start chat.” Trace how the failure affects the first-message prompt and encryption notice; done when the UI no longer presents in-room prompts or the encryption message while chat creation has failed.

Written by the indexing model from the issue text.