Distinguish sign-out from console API failure

[albertovincenzi]

Problem

The authentication bootstrap state machine says that only a 401 means the user must log in, but fetchMe caught every error and switched to the login screen. A network failure or a console/API 5xx therefore misleadingly offered “Continue with Google” even though the session status was unknown; starting OAuth again cannot repair a backend outage.

Fix

• add an explicit authentication error state and retain its diagnostic message
• keep the login transition exclusive to a real 401 response
• render a “Console unavailable” panel for bootstrap failures
• provide an in-place retry that clears the error after recovery

Verification

• npm run build
• npm audit (0 vulnerabilities)
• git diff --check master...HEAD

Review notes

This is a frontend-only, independently mergeable fix based on master. It complements the API read-error visibility work in #63, but does not depend on it.

[alice-viola]

Landed on master via #67 (merge commit 944ee9b) as part of the 62-PR integration — this PR's head commit d5f2748 is an ancestor of master. GitHub could not mark it merged automatically because its base is fix/protect-logout, not master. Closing as landed.