Preview Mode: Error Modal + Improve Error Dialog when preview can't reach local server

### Description

When previewing a local scene on mobile (via QR code / `npm run start -- --mobile`), if the phone cannot reach the computer over the local network (e.g. hotel/guest WiFi that isolates clients from seeing each other), the app currently just hangs — the loading screen never leaves and there's no feedback telling the user what's wrong or how to fix it.

### Context

Came out of a thread in #mobile-support where `pravus` reported local scene preview "broken" on Android — QR scan worked, but the scene never loaded. Production/deployed scenes loaded fine on the same device, isolating the problem to local scene preview specifically. Root cause was traced to the phone's network (hotel WiFi) blocking client-to-client local IP visibility, not a client bug — desktop → phone connectivity for local dev scenes depends on being able to reach the PC's local IP directly.

### Proposal

- Detect when the local scene preview server isn't reachable (timeout while connecting) instead of hanging indefinitely on the loading screen.
- Show an error dialog explaining the phone can't reach the computer, with guidance (e.g. "make sure your phone and computer are on the same WiFi network and that the network allows devices to see each other — some hotel/public/guest networks block this").

### Open question

Worth tracking how many users hit this to gauge whether we need a more robust solution than local IP sharing (e.g. tunneling) — cc Gonzalo.

Requested by Mateo via Slack