fix(frontend): unregister auth error handler on AuthProvider unmount (#1070)

Issue #579 reported that page reload did not redirect to /login when the
session expired. Root-cause tracing showed the reload guard already works
on development (all 16 auth E2E tests pass), but the trace surfaced a real
hazard: the axios 401 interceptor kept a stale handler closure after
AuthProvider unmounted. setAuthErrorHandler now accepts null and the
provider unregisters its handler on unmount.

Adds regression tests for RequireAuth, AuthContext mount/unmount handler
lifecycle, and the client interceptor null-handler path.

Closes #579

Co-authored-by: GitHub Actions <actions@github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people