fix(store): keep the local user unless the session check returns 401

whoami() told connection failures apart from a missing session by looking
for "Failed to fetch" in the error message. Electron's IPC fetch, React
Native's fetch and SDK timeouts use different messages, so an unreachable
API cleared the local user, and on mobile redirected to the login screen.

Only a 401 (or an empty session) now clears the user. Every other failure
is rethrown so callers keep the cached user.

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