[Bug] Mobile Preview QR requires double scan on Windows (Bevy headless server)

### Platform
Windows (desktop preview host); mobile client platform not specified

### App Version
Not specified

### Device Information
Not specified

### Issue Description
Mobile Preview Mode QR code requires scanning twice before the scene loads, on Windows. This reproduces even now that scenes use the Bevy headless server (which auto-starts with the scene, no separate manual launch step required).

A closely related bug (#1836) was previously fixed: it was caused by the old LiveKit auth server not auto-starting on Windows, requiring a manual launch in a separate console — scanning before that server finished connecting caused a stuck 0% load, and a second scan always worked once it had connected. That fix does not appear to cover the current Bevy-headless-server path, since the auto-start step it addressed no longer applies but the double-scan symptom persists.

### Expected Behavior
Scanning the Preview Mode QR code once should load the scene, without needing a second scan.

### Steps to Reproduce
1. Run a scene preview in Creator Hub / SDK CLI on Windows (Bevy headless server, auto-started).
2. Scan the Preview Mode QR code with the mobile app.
3. Observe the scene fails to load / gets stuck on first scan.
4. Scan the same QR code a second time.
5. Scene loads successfully on the second scan.

### Occurrence
Reported as consistently reproducible on Windows (exact percentage not specified).

### References
- #1836 — prior, related, and fixed "stuck at 0% on first scan" issue for the old manually-launched LiveKit auth server; root cause does not fully explain this occurrence since the Bevy server auto-starts.

Requested by Eibriel via Slack