feat: show a prompt when the API cannot be reached

Requests that fail without any response (DNS failure, refused or reset
connection, offline device, timeout) previously failed silently. Track
them through the SDK interceptors and surface a single prompt per outage:
a persistent toast on desktop that is dismissed once the API answers
again, and a transient toast on mobile.

The connection state lives in an atom because the first requests can fail
before the toaster is mounted.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>